doc: typo fix for net/http.ServeMux

Updates golang#61422

Change-Id: I2d86863db70792122b5168935e76d9ca38f761b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/551256
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Auto-Submit: Carlos Amedee <[email protected]>

Author: cagedmantis

doc/go1.22.html

@@ -359,7 +359,7 @@ <h3 id="math_rand_v2">New math/rand/v2 package</h3>
 We plan to include an API migration tool in a future release, likely Go 1.23.
 </p>
 
-<h3 id="enhanced_routing_patterns"</h3>
+<h3 id="enhanced_routing_patterns">Enhanced routing patterns</h3>
 
 <p><!-- https://go.dev/issue/61410 -->
   HTTP routing in the standard library is now more expressive.