doc/go1.12: correct types for math/bits

Extended precision math/bits functions are unsigned.

Change-Id: Ic1633e9c367fc3d5a80bc503008f035db4e78945
Reviewed-on: https://go-review.googlesource.com/c/155379Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -547,7 +547,7 @@ for {
 <dl id="math/bits"><dt><a href="/pkg/math/bits/">math/bits</a></dt>
   <dd>
     <p><!-- CL 123157 -->
-    New extended precision operations <a href="/pkg/math/bits/#Add"><code>Add</code></a>, <a href="/pkg/math/bits/#Sub"><code>Sub</code></a>, <a href="/pkg/math/bits/#Mul"><code>Mul</code></a>, and <a href="/pkg/math/bits/#Div"><code>Div</code></a> are available in <code>int</code>, <code>int32</code>, and <code>int64</code> versions.
+    New extended precision operations <a href="/pkg/math/bits/#Add"><code>Add</code></a>, <a href="/pkg/math/bits/#Sub"><code>Sub</code></a>, <a href="/pkg/math/bits/#Mul"><code>Mul</code></a>, and <a href="/pkg/math/bits/#Div"><code>Div</code></a> are available in <code>uint</code>, <code>uint32</code>, and <code>uint64</code> versions.
     </p>
 
 </dl><!-- math/bits -->