rpc: fixed comment grammar issue (#17146)

rpc/client.go

@@ -304,7 +304,7 @@ func (c *Client) CallContext(ctx context.Context, result interface{}, method str
 		return err
 	}
 
-	// dispatch has accepted the request and will close the channel it when it quits.
+	// dispatch has accepted the request and will close the channel when it quits.
 	switch resp, err := op.wait(ctx); {
 	case err != nil:
 		return err