[gdb/contrib] Fix shellcheck warnings in spellcheck.sh

Fix shellcheck warnings in spellcheck.sh, found using shellcheck v0.10.0.

Ran shellcheck v0.10.0 (on a system with shellcheck version 0.8.0) using this
command from an RFC patch [1]:
...
$ ./gdb/contrib/pre-commit-shellcheck.sh ./gdb/contrib/spellcheck.sh
...

Tested on x86_64-linux

[1] https://sourceware.org/pipermail/gdb-patches/2024-November/213400.html
1 file changed