responsive CSS fix (#1533)

Your player is great, thank you! I found a syntax error that breaks some of the responsive CSS. This tiny edit -- adding a single semicolon to one line of CSS -- fixes the problem.

README.md

@@ -211,7 +211,7 @@ class ResponsivePlayer extends Component {
 ```css
 .player-wrapper {
   position: relative;
-  padding-top: 56.25% /* Player ratio: 100 / (1280 / 720) */
+  padding-top: 56.25%; /* Player ratio: 100 / (1280 / 720) */
 }
 
 .react-player {