doc/go1.14: fix minor typo (update release notes)

Follow-up on https://golang.org/cl/216200/2/doc/go1.14.html#423 .

Updates #36878.

Change-Id: I693a9eb05c6f1f42721a92fda46a4f3449defa24
Reviewed-on: https://go-review.googlesource.com/c/go/+/217100
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.14.html

@@ -499,7 +499,7 @@ <h2 id="library">Core library</h2>
 <dl id="math/big"><dt><a href="/pkg/math/big">math/big</a></dt>
   <dd>
     <p><!-- CL 164972 -->
-      The <a href="/pkg/math/big#GCD"><code>GCD</code></a> function
+      The <a href="/pkg/math/big/#GCD"><code>GCD</code></a> function
       now allows the inputs <code>a</code> and <code>b</code> to be
       zero or negative.
     </p>