context: fix typo in TODO documentation

This fixes an incorrect contraction, and makes it easier to read.

Change-Id: I8b88b6f54df25c4c36e00e6fbe77b3c32e518e12
Reviewed-on: https://go-review.googlesource.com/16895
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

context/context.go

@@ -189,7 +189,7 @@ func Background() Context {
 }
 
 // TODO returns a non-nil, empty Context.  Code should use context.TODO when
-// it's unclear which Context to use or it's is not yet available (because the
+// it's unclear which Context to use or it is not yet available (because the
 // surrounding function has not yet been extended to accept a Context
 // parameter).  TODO is recognized by static analysis tools that determine
 // whether Contexts are propagated correctly in a program.