doc: fix broken link in go1.8.html

Fixes #19244

Change-Id: Ia6332941b229c83d6fd082af49f31003a66b90db
Reviewed-on: https://go-review.googlesource.com/37388Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go1.8.html

@@ -1304,7 +1304,7 @@ crypto/x509: return error for missing SerialNumber (CL 27238)
         request must have the new
         <a href="/pkg/net/http/#Request"><code>Request.GetBody</code></a>
         field defined.
-        <a href="pkg/net/http/#NewRequest"><code>NewRequest</code></a>
+        <a href="/pkg/net/http/#NewRequest"><code>NewRequest</code></a>
         sets <code>Request.GetBody</code> automatically for common
         body types.
       </li>