doc/effective_go: fix grammatical error

Change-Id: Ib52854169f3dd18f54d3ae9263c897d3f45ea869
Reviewed-on: https://go-review.googlesource.com/10982Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/effective_go.html

@@ -3054,7 +3054,7 @@ req := req
 </pre>
 
 <p>
-but it's a legal and idiomatic in Go to do this.
+but it's legal and idiomatic in Go to do this.
 You get a fresh version of the variable with the same name, deliberately
 shadowing the loop variable locally but unique to each goroutine.
 </p>