context: use https in docs

Change-Id: I9354712768702e3b083c77f30165a34cb414d686
Reviewed-on: https://go-review.googlesource.com/22810Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/context.go

@@ -32,7 +32,7 @@
 // The same Context may be passed to functions running in different goroutines;
 // Contexts are safe for simultaneous use by multiple goroutines.
 //
-// See http://blog.golang.org/context for example code for a server that uses
+// See https://blog.golang.org/context for example code for a server that uses
 // Contexts.
 package context
 
@@ -81,7 +81,7 @@ type Context interface {
 	//  	}
 	//  }
 	//
-	// See http://blog.golang.org/pipelines for more examples of how to use
+	// See https://blog.golang.org/pipelines for more examples of how to use
 	// a Done channel for cancelation.
 	Done() <-chan struct{}