Fix i2c docs formatting

[michaelgruenewald]

Make sure all boxes are checked (add x inside the brackets) when you submit your contribution, remove this sentence before doing so.

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

This just fixes the formatting of the initial bullet point list in the i2c module documentation so that mkdocs actually recognizes it as a list.

[HHHartmann]

Thanks for your contribution. That was one of the errors which are often not found by the developer because it all looks good on github.