doc: add missing p in install from source page

The last paragraph in golang.org/doc/install/source#fetch is missing a
p tag, so it doesn't get formatted with the 'max-width: 50rem' like
all the other text in the page.

Add it.

Change-Id: I1a981dd2afde561b4ab21bd90ad99b3a146111f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/210122Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -247,10 +247,12 @@ that if Go is checked out in <code>$HOME/go</code>, it will conflict with
 the default location of <code>$GOPATH</code>.
 See <a href="#gopath"><code>GOPATH</code></a> below.</p>
 
+<p>
 Reminder: If you opted to also compile the bootstrap binaries from source (in an
 earlier section), you still need to <code>git clone</code> again at this point
 (to checkout the latest <code>&lt;tag&gt;</code>), because you must keep your
 go1.4 repository distinct.
+</p>
 
 <h2 id="head">(Optional) Switch to the master branch</h2>