doc/go1.17: clean up formatting of gofmt section

It was the only h3 in <code>, and it lacked <p> around its content.

It looked like it was part of the prior section:

    https://tip.golang.org/doc/go1.17#gofmt

Change-Id: I7e9ef70e9a03474225833f44420aabd70dab3cd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/326774


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>