fix footer partial name

[lionralfs]

What's changed, or what was fixed?

• Inconsistency in partial name. In the first few code samples, the "partial includes" follow the <partial-name>.include naming pattern. For reference, here is the live article.

• This has been reviewed and approved by (NAME)

• I have run npm test locally and all tests pass.

• I have added the appropriate type-something label.

• I've staged the site and manually verified that my content displays correctly.

• I've done neither of these ^ things as I've just edited directly via GitHub

CC: @petele