[gdb/contrib] Handle capitalized words in spellcheck.sh

[gdb/contrib] Handle capitalized words in spellcheck.sh

The dictionary contains a few entries with capital letters:
...
$ grep -E '[A-Z]' .git/wikipedia-common-misspellings.txt | wc -l
143
...
but they don't look too interesting in the gdb context (for instance,
Habsbourg->Habsburg), so filter them out.

That leaves us with entries looking only like "foobat->foobar", so add
handling of capitalized words, such that we also rewrite "Foobat" to "Foobar".

Tested on aarch64-linux.  Verified with shellcheck.

Approved-by: Kevin Buettner <kevinb@redhat.com>