tools/gen-cpydiff.py: Adjust subsections to sentence case.

tools/gen-cpydiff.py

@@ -51,7 +51,7 @@ INDEX = 'index.rst'
 
 HEADER = '.. This document was generated by tools/gen-cpydiff.py\n\n'
 UIMPORTLIST = {'struct', 'collections', 'json'}
-CLASSMAP = {'Core': 'Core Language', 'Types': 'Builtin Types'}
+CLASSMAP = {'Core': 'Core language', 'Types': 'Builtin types'}
 INDEXPRIORITY = ['syntax', 'core_language', 'builtin_types', 'modules']
 RSTCHARS = ['=', '-', '~', '`', ':']
 SPLIT = '"""\n|categories: |description: |cause: |workaround: '