doc: clarify to use MinGW-W64 for amd64 systems

Fixes #19686

Change-Id: I5c2dd60636b521425647afd0725fdd7c18e7bbbe
GitHub-Last-Rev: a1c5f569409d8759d5d81da811eaf873073bfe39
GitHub-Pull-Request: golang/go#31682
Reviewed-on: https://go-review.googlesource.com/c/go/+/173997Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install.html

@@ -50,7 +50,7 @@ If your OS or architecture is not on the list, you may be able to
 <tr><td>FreeBSD 10.3 or later</td> <td>amd64, 386</td> <td>Debian GNU/kFreeBSD not supported</td></tr>
 <tr valign='top'><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm, arm64,<br>s390x, ppc64le</td> <td>CentOS/RHEL 5.x not supported.<br>Install from source for other libc.</td></tr>
 <tr><td>macOS 10.10 or later</td> <td>amd64</td> <td>use the clang or gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup> for <code>cgo</code> support</td></tr>
-<tr><td>Windows 7, Server 2008R2 or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
+<tr valign='top'><td>Windows 7, Server 2008R2 or later</td> <td>amd64, 386</td> <td>use MinGW (<code>386</code>) or MinGW-W64 (<code>amd64</code>) gcc<sup>&#8224;</sup>.<br>No need for cygwin or msys.</td></tr>
 </table>
 
 <p>