DOC: Update links to plots in README.rst after switching default git …

…branch from 'master' to 'main'.

README.rst

@@ -43,7 +43,7 @@ Example 1
 
 The following script creates this PNG file, an 8-bit RGB image.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example1.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example1.png
    :alt: Example 1
    :align: center
 
@@ -75,7 +75,7 @@ Example 2
 
 The following script creates this PNG file, a 1-bit grayscale image.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example2.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example2.png
    :alt: Example 2
    :align: center
 
@@ -106,7 +106,7 @@ The following script creates this PNG file, a 16-bit RGB file in which
 the value (0, 0, 0) is transparent.  It might not be obvious, but the
 two squares are transparent.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example3.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example3.png
    :alt: Example 3
    :align: center
 
@@ -147,7 +147,7 @@ Example 4
 The following script uses the option ``use_palette=True`` to create this 8-bit
 indexed RGB file.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example4.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example4.png
    :alt: Example 4
    :align: center
 
@@ -182,7 +182,7 @@ Example 5
 
 This animated PNG
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example5.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example5.png
    :alt: Example 5
    :align: center
 
@@ -222,7 +222,7 @@ Another animated RGB PNG. In this example, the argument ``seq``
 that is passed to ``write_apng`` is a numpy array with shape
 ``(num_frames, height, width, 3)``.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example6.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example6.png
    :alt: Example 6
    :align: center
 
@@ -263,7 +263,7 @@ Example 7
 
 Create an animated PNG with different display times for each frame.
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example7.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example7.png
    :alt: Example 7
    :align: center
 
@@ -395,7 +395,7 @@ This example shows how a Matplotlib animation can be saved as
 an animated PNG file with `numpngw.AnimatedPNGWriter`.  (Be careful
 with this class--it can easily create very large PNG files.)
 
-.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/master/examples/example8.png
+.. image:: https://raw.githubusercontent.com/WarrenWeckesser/numpngw/main/examples/example8.png
    :alt: Example 8
    :align: center