Fix misleading explanation of comma in $(),*

The previous explanation makes it sound like the comma in that place would match an optional trailing comma like in `[a, b, c,]`. That is incorrect, and actually that only matches commas between elements.

src/ch19-06-macros.md

@@ -119,8 +119,9 @@ for use in the replacement code. Within `$()` is `$x:expr`, which matches any
 Rust expression and gives the expression the name `$x`.
 
 The comma following `$()` indicates that a literal comma separator character
-could optionally appear after the code that matches the code in `$()`. The `*`
-specifies that the pattern matches zero or more of whatever precedes the `*`.
+must appear between each instance of the code that matches the code within 
+`$()`. The `*` specifies that the pattern matches zero or more of whatever 
+precedes the `*`.
 
 When we call this macro with `vec![1, 2, 3];`, the `$x` pattern matches three
 times with the three expressions `1`, `2`, and `3`.