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>

doc/go1.17.html

@@ -293,15 +293,17 @@ <h4 id="build-lines"><code>//go:build</code> lines</h4>
   <a href="https://golang.org/design/draft-gobuild">https://golang.org/design/draft-gobuild</a>.
 </p>
 
-<h3 id="gofmt"><code>gofmt</code></h3>
+<h3 id="gofmt">Gofmt</h3>
+
+<p>
   <code>gofmt</code> (and <code>go</code> <code>fmt</code>) now synchronizes
   <code>//go:build</code> lines with <code>// +build</code> lines. If a file
   only has <code>// +build</code> lines, they will be moved to the appropriate
   location in the file, and matching <code>//go:build</code> lines will be
   added. Otherwise, <code>// +build</code> lines will be overwritten based on
   any existing <code>//go:build</code> lines. For more information, see
   <a href="https://golang.org/design/draft-gobuild">https://golang.org/design/draft-gobuild</a>.
-</h3>
+</p>
 
 <h3 id="vet">Vet</h3>