Effective Go: fix typo

Fixes #1467.

R=r, r2
CC=golang-dev
https://golang.org/cl/3981054

doc/effective_go.html

@@ -2705,7 +2705,7 @@ func init() {
 
 <p>
 When <code>panic</code> is called, including implicitly for run-time
-errors such indexing an array out of bounds or failing a type
+errors such as indexing an array out of bounds or failing a type
 assertion, it immediately stops execution of the current function
 and begins unwinding the stack of the goroutine, running any deferred
 functions along the way.  If that unwinding reaches the top of the