[mlir][doc] Fix the EBNF description of mlir syntax in language reference doc

According to the EBNF syntax described in the 'Common syntax' chapter,
literal characters should be surrounded by backticks (`). However, in
some sections of this document, single quotes (') are used instead.
So, fix them.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D150067