Use relative links in README table of contents

[injust]

When reading the README on https://github.com/MitMaro/git-interactive-rebase-tool, this avoids an unnecessary page navigation to https://github.com/MitMaro/git-interactive-rebase-tool/blob/master/README.md.

Also fixed a broken link (#usage -> #direct-usage).

[MitMaro]

Looks good! Thanks for the updates!

Assuming CI passes (which it should since this is only README updates), I will merge. :)

[coveralls]

Pull Request Test Coverage Report for Build 10225304537

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.323%

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Totals
Change from base Build 10032197327:	0.0%
Covered Lines:	4581
Relevant Lines:	4707

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💛 - Coveralls