_content/doc/install: update GOARM default on non-arm systems

As of Go 1.21, the default GOARM value when cross-compiling is 7.
This was done in CL 470695. Update its documentation accordingly.

For golang/go#24904.
For golang/go#65241.

Change-Id: I04a22b1e6d5b324615b4822f18db3c53270cf4fb
Reviewed-on: https://go-review.googlesource.com/c/website/+/560015
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

_content/doc/install/source.html

@@ -703,7 +703,7 @@ <h2 id="environment">Optional environment variables</h2>
 </li>
 
 <li><code>$GOARM</code> (for <code>arm</code> only; default is auto-detected if building
-on the target processor, 6 if not)
+on the target processor, 7 if not)
 <p>
 This sets the ARM floating point co-processor architecture version the run-time
 should target. If you are compiling on the target system, its value will be auto-detected.