Update corewar/index.md

[robgorham]

Links are broken without .md suffix for Markdown files

[dougajmcdonald]

Hi @robgorham thanks for the PR, I was just checking this PR and wondering whether to merge it.

When I check https://corewar-docs.readthedocs.io/en/latest/corewar/ and click on the link to the intro bits you've edited, the rendered links (e.g. https://corewar-docs.readthedocs.io/en/latest/corewar/warriors ) seem to link correctly to the live docs pages.

You might want to check with @gareththegeek whether there is a convention in MKDocs which translates the links when deployed, but at the moment, things seem to work Ok on prod.

Was there a specific link/area/environment you were seeing a problem?