_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 <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/_content/doc/install/source.html b/_content/doc/install/source.html
index a3cc582..54ff166 100644
--- a/_content/doc/install/source.html
+++ b/_content/doc/install/source.html
@@ -703,7 +703,7 @@
 </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.