doc/go1.12: fix GOARCH value in Syscall18 link

Currently the link works also with the non-existing GOARCH armd64, but
let's correct in anyhow.

Change-Id: Ida647b8f9dd2f8460b019f5a23759f10a6da8e60
Reviewed-on: https://go-review.googlesource.com/c/155277Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -738,7 +738,7 @@ for {
     </p>
 
     <p><!-- CL 147117 -->
-      The new function  <a href="/pkg/syscall/?GOOS=windows&GOARCH=armd64#Syscall18"><code>Syscall18</code></a>
+      The new function  <a href="/pkg/syscall/?GOOS=windows&GOARCH=amd64#Syscall18"><code>Syscall18</code></a>
       has been introduced for Windows, allowing for calls with up to 18 arguments.
     </p>