_content/code.html: clarify that 'go test' runs in the source directory

Fixes golang/go#44460.

Change-Id: I8192bffa691015b1cb9a3cfa78958850891a67e5
Reviewed-on: https://go-review.googlesource.com/c/website/+/350250
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/_content/doc/code.html b/_content/doc/code.html
index 973871a..a6ba8c2 100644
--- a/_content/doc/code.html
+++ b/_content/doc/code.html
@@ -413,6 +413,7 @@
 </p>
 
 <pre>
+$ cd $HOME/hello/morestrings
 $ <b>go test</b>
 PASS
 ok  	example/user/hello/morestrings 0.165s