_content/doc/go1.21: correct grammar error

Fixes golang/go#61854

Change-Id: Ia53bb95150901ccecf038962577e9eaa30755f16
Reviewed-on: https://go-review.googlesource.com/c/website/+/517255
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/_content/doc/go1.21.html b/_content/doc/go1.21.html
index ee4c837..003ce4c 100644
--- a/_content/doc/go1.21.html
+++ b/_content/doc/go1.21.html
@@ -154,7 +154,7 @@
   To support programs written for older versions of Go, nil panics can be re-enabled by setting
   <code>GODEBUG=panicnil=1</code>.
   This setting is enabled automatically when compiling a program whose main package
-  is in a module with that declares <code>go</code> <code>1.20</code> or earlier.
+  is in a module that declares <code>go</code> <code>1.20</code> or earlier.
 </p>
 
 <h2 id="tools">Tools</h2>