update repos example to use RateLimits

The RateLimit() func is deprecated, and should not appear in the
official example.

examples/repos/main.go

@@ -24,7 +24,7 @@ func main() {
 		fmt.Printf("%v\n\n", github.Stringify(repos))
 	}
 
-	rate, _, err := client.RateLimit()
+	rate, _, err := client.RateLimits()
 	if err != nil {
 		fmt.Printf("Error fetching rate limit: %#v\n\n", err)
 	} else {

github/github.go

@@ -66,7 +66,7 @@ type Client struct {
 
 	// Rate specifies the current rate limit for the client as determined by the
 	// most recent API call.  If the client is used in a multi-user application,
-	// this rate may not always be up-to-date.  Call RateLimit() to check the
+	// this rate may not always be up-to-date.  Call RateLimits() to check the
 	// current rate.
 	Rate Rate