[gdb/contrib] Handle parentheses in spellcheck.sh

Currently, text adjacent to parentheses is not spell-checked:
...
$ cat tmp.txt
(upto)
$ ./gdb/contrib/spellcheck.sh tmp.txt
$
...

Add handling of parentheses, such that we get:
...
$ ./gdb/contrib/spellcheck.sh tmp.txt
upto -> up to
$
...

Rerun spellcheck.sh, resulting in a few "thru->through" replacements.

Tested on x86_64-linux.

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