context: fix typo in comments

Change-Id: I41310ec88c889fda79d80eaf4a742a1000284f60
Reviewed-on: https://go-review.googlesource.com/23591Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/context/context.go

@@ -107,7 +107,7 @@ type Context interface {
 	// collisions.
 	//
 	// Packages that define a Context key should provide type-safe accessors
-	// for the values stores using that key:
+	// for the values stored using that key:
 	//
 	// 	// Package user defines a User type that's stored in Contexts.
 	// 	package user