doc: add some missing HTML tags in the FAQ

Fixes #17170

Change-Id: I939f087df133710495fdf6f09040051cb9b176d7
Reviewed-on: https://go-review.googlesource.com/29442Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_faq.html

@@ -889,6 +889,7 @@ Why does Go not have covariant result types?</h3>
 
 <p>
 Covariant result types would mean that an interface like
+</p>
 
 <pre>
 type Copyable interface {
@@ -896,7 +897,9 @@ type Copyable interface {
 }
 </pre>
 
+<p>
 would be satisfied by the method
+</p>
 
 <pre>
 func (v Value) Copy() Value