doc/go1.21: fix link in WTF-8 section

Change-Id: I8fbba7ca5a148ff6523be412991af0c089e7f7ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/499516
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: tklauser

doc/go1.21.html

@@ -933,7 +933,7 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
       and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
       functions now convert between UTF-16 data and
-      <a href=http://simonsapin.github.io/wtf-8/"">WTF-8</a> strings.
+      <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
       This is backward compatible as WTF-8 is a superset of the UTF-8
       format that was used in earlier releases.
     </p>