[flang] OpenMP 4.5 grammar related minor fixes

1. typo and trailing spaces within OpenMP-4.5-grammar.txt

2. parse (openmp-grammar.h) and unparse(unparse.cc) for:

   SCHEDULE ([modifier [, modifier]:]kind[, chunk_size])
   Modifier ->  MONITONIC | NONMONOTONIC | SIMD

where ":" is optional except at least one modifier is present.

Original-commit: flang-compiler/f18@5cebae50390a1b22316ef2d86e87f2dabaaaa61f
Reviewed-on: https://github.com/flang-compiler/f18/pull/453