Fix a few broken links in docs

I noticed some broken links while browsing the capybara manual (here). This PR fixes them.

Perhaps capybara's default branch used to be main? I'm not sure... but presently it is master, and so these links need to be updated.

Before submitting the PR make sure the following are checked:

[x] Feature branch is up-to-date with main (if not - rebase it).
[x] Squashed related commits together.
[x] Added tests.
[x] Updated documentation.
[x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
[x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

[ ] ~Added the new cop to config/default.yml.~
[ ] ~The cop is configured as Enabled: pending in config/default.yml.~
[ ] ~The cop documents examples of good and bad code.~
[ ] ~The tests assert both that bad code is reported and that good code is not reported.~
[ ] ~Set VersionAdded: "<<next>>" in default/config.yml.~

If you have modified an existing cop's configuration options:

rubocop / rubocop-capybara