doc: website: changed link for cover

Changed link for cover from x-tools to correct

Fix #20662

Change-Id: I9b839ed952e9abb12b3d1655ac4cf5976f374a4b
Reviewed-on: https://go-review.googlesource.com/46012Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/cmd.html

@@ -62,7 +62,7 @@ details.
 </tr>
 
 <tr>
-<td><a href="//godoc.org/golang.org/x/tools/cmd/cover/">cover</a></td>
+<td><a href="/cmd/cover/">cover</a></td>
 <td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
 <td>Cover is a program for creating and analyzing the coverage profiles
 generated by <code>"go test -coverprofile"</code>.</td>