doc: fix broken link to the vet command documentation

Fixes #15188

Change-Id: I0ab7791f7db499cef6bc52292d3d93ff4da7caff
Reviewed-on: https://go-review.googlesource.com/23151Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/cmd.html

@@ -89,7 +89,7 @@ gofmt</a> command with more general options.</td>
 </tr>
 
 <tr>
-<td><a href="//godoc.org/golang.org/x/tools/cmd/vet/">vet</a></td>
+<td><a href="/cmd/vet/">vet</a></td>
 <td>&nbsp;&nbsp;&nbsp;&nbsp;</td>
 <td>Vet examines Go source code and reports suspicious constructs, such as Printf
 calls whose arguments do not align with the format string.</td>