Fixing syntax highlighting in some docs

src/dialog/README.md

@@ -25,7 +25,7 @@ After your dialog markup is created, add the event listeners to the page to trig
 
 For example:
 
-```js
+```javascript
   var button = document.querySelector('button');
   var dialog = document.querySelector('dialog');
   button.addEventListener('click', function() {

src/snackbar/README.md

@@ -46,7 +46,7 @@ All snackbars should be shown through the same element.
 
 ### Snackbar
 
-```js
+```javascript
 var notification = document.querySelector('.mdl-js-snackbar');
 var data = {
   message: 'Message Sent',
@@ -59,7 +59,7 @@ notification.MaterialSnackbar.showSnackbar(data);
 
 ### Toast
 
-```js
+```javascript
 var notification = document.querySelector('.mdl-js-snackbar');
 notification.MaterialSnackbar.showSnackbar(
   {