[llvm-dwarfdump] Simplify --ignore-case help text and documentation

There was a typo in the --ignore-case help text that was copied into the
llvm-dwarfdump command-guide. Additionally, this patch simplifies the
wording, since it was unnecessarily verbose: the switch applies for
searching in general and doesn't need explicitly stating different
search modes (which might go out-of-date as options are added or
removed).

Reviwed by: JDevlieghere

Differential Revision: https://reviews.llvm.org/D63133

llvm-svn: 363066