Fix a broken link

[hugovk]

For #681.

$ make linkcheck | grep broken
(developer-workflow/grammar: line   40) broken    https://github.com/python/cpython/blob/main/Include/Python-ast.h - 404 Client Error: Not Found for url: https://github.com/python/cpython/blob/main/Include/Python-ast.h
(internals/compiler: line  516) broken    https://github.com/python/cpython/blob/main/Include/code.h - 404 Client Error: Not Found for url: https://github.com/python/cpython/blob/main/Include/code.h
(developer-workflow/grammar: line   33) broken    https://github.com/python/cpython/blob/main/Include/token.h - 404 Client Error: Not Found for url: https://github.com/python/cpython/blob/main/Include/token.h
(internals/compiler: line  488) broken    https://github.com/python/cpython/blob/main/Python/peephole.c - 404 Client Error: Not Found for url: https://github.com/python/cpython/blob/main/Python/peephole.c
(internals/compiler: line  586) broken    https://github.com/python/cpython/blob/main/Python/wordcode_helpers.h - 404 Client Error: Not Found for url: https://github.com/python/cpython/blob/main/Python/wordcode_helpers.h
(advanced-tools/gdb: line  355) broken    https://sourceware.org/gdb/onlinedocs/gdb/Specify-Location.html - 404 Client Error: Not Found for url: https://sourceware.org/gdb/onlinedocs/gdb/Specify-Location.html
make: *** [linkcheck] Error 1

The first five are covered by #1003.

This PR fixes the last one.

Old page:

https://web.archive.org/web/20220616003657/https://sourceware.org/gdb/onlinedocs/gdb/Specify-Location.html

New page:

https://sourceware.org/gdb/onlinedocs/gdb/Location-Specifications.html