doc/go1.11: update runtime/trace to use region instead of span

The final API uses 'region' instead of 'span' from the proposal.

Change-Id: I305da891a360596fff89b10bc6de3090289b5396
Reviewed-on: https://go-review.googlesource.com/124815Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.11.html

@@ -309,7 +309,7 @@ func f(v interface{}) {
   in execution traces and create groups of related goroutines.
   The <code>go</code>&nbsp;<code>tool</code>&nbsp;<code>trace</code>
   command visualizes this information in the trace view and the new
-  user task/span analysis page.
+  user task/region analysis page.
 </p>
 
 <h2 id="runtime">Runtime</h2>