doc/go1.13: fix bad URLs to strconv identifiers

Change-Id: I7db3ad060773c9396fbe34e7bd52e7ccf6e5e52c
Reviewed-on: https://go-review.googlesource.com/c/go/+/191797Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.13.html

@@ -918,19 +918,19 @@ godoc
 
 </dl><!-- runtime -->
 
-<dl id="strconv"><dt><a href="/pkg/strconv">strconv</a></dt>
+<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
   <dd>
     <p><!-- CL 160243 -->
-       For <a href="/pkg/math/big/#strconv.ParseFloat"><code>strconv.ParseFloat</code></a>,
-       <a href="/pkg/math/big/#strconv.ParseInt"><code>strconv.ParseInt</code></a>
-       and <a href="/pkg/math/big/#strconv.ParseUint"><code>strconv.ParseUint</code></a>,
+       For <a href="/pkg/strconv/#ParseFloat"><code>strconv.ParseFloat</code></a>,
+       <a href="/pkg/strconv/#ParseInt"><code>strconv.ParseInt</code></a>
+       and <a href="/pkg/strconv/#ParseUint"><code>strconv.ParseUint</code></a>,
        if base is 0, underscores may be used between digits for readability.
        See the <a href="#language">Changes to the language</a> for details.
     </p>
 
 </dl><!-- strconv -->
 
-<dl id="strings"><dt><a href="/pkg/strings">strings</a></dt>
+<dl id="strings"><dt><a href="/pkg/strings/">strings</a></dt>
   <dd>
     <p><!-- CL 142003 -->
       The new <a href="/pkg/strings/#ToValidUTF8"><code>ToValidUTF8</code></a> function returns a