bump 3.12 to 3.13 in readme (#1228)

Author: khameeteman

README.rst

@@ -33,7 +33,7 @@ A Concrete Syntax Tree (CST) parser and serializer library for Python
 
 .. intro-start
 
-LibCST parses Python 3.0 -> 3.12 source code as a CST tree that keeps
+LibCST parses Python 3.0 -> 3.13 source code as a CST tree that keeps
 all formatting details (comments, whitespaces, parentheses, etc). It's useful for
 building automated refactoring (codemod) applications and linters.