Fix an incorrect link in external resources page
Below is a curated collection of external PyGMT resources.
-*To add your contribution to this collection, follow [these instructions](https://github.com/GenericMappingTools/pygmt/blob/main/CONTRIBUTING.md#editing-the-documentation)
+*To add your contribution to this collection, follow [these instructions](contributing.md#contributing-documentation)
to submit a pull request with your recommended addition to the
[External Resources file](https://github.com/GenericMappingTools/pygmt/blob/main/doc/external_resources.md).*
