mkindex.py does not index grammars with preprocessor grammars correctly

[kaby76]

https://github.com/antlr/grammars-v4/blob/f46e731fa5676e2768fe92f58a475fbe6870e21a/_scripts/mkindex.py#L97

For Cobol85, the lexer and parser grammar is set to the preprocessor grammar instead of the main grammar.

Found when trying to test https://github.com/antlr/grammars-v4/issues/2904