fix: Use selector argument in make_example

All of the charts using this function were silently producing no output.
`brush` had been defined globally in an earlier directive

vega#3544 (comment)

doc/user_guide/interactions/parameters.rst

@@ -318,7 +318,7 @@ selection:
         return alt.Chart(cars).mark_rect().encode(
             x="Cylinders:O",
             y="Origin:N",
-            color=alt.when(brush).then("count()").otherwise(alt.value("lightgray")),
+            color=alt.when(selector).then("count()").otherwise(alt.value("lightgray")),
         ).properties(
             width=300,
             height=180