mime: add missing path for mime types in godoc

mime: add missing path for mime types in godoc

ExtensionsByType was missing godoc for a mime.types file path
that was added in CL 236677.

Change-Id: Ia04fa90f01490fc1f063c4905aea65d240935cab
Reviewed-on: https://go-review.googlesource.com/c/go/+/721580
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>