cmd/go/internal/modindex: remove spurious field from index_format documentation

cmd/go/internal/modindex: remove spurious field from index_format documentation

The 'path' field was removed in an earlier revision to the format.

While auditing the format, I also cleaned up a couple of minor
typographical issues.

For #53586.

Change-Id: I4cd1ce9e970023441c11244428ed2971be1d8138
Reviewed-on: https://go-review.googlesource.com/c/go/+/415514
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>