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>