docs(mdPanel): fix quote char, &s, unused var

* Fix invalid quotation mark (‘’ -> '').
* Fix escaped & problem (&) by changing syntax.
* Remove unused variable.

Author: zbjornson

src/components/panel/panel.js

@@ -26,7 +26,7 @@ angular
  * @usage
  * <hljs lang="js">
  * (function(angular, undefined) {
- *   ‘use strict’;
+ *   'use strict';
  *
  *   angular
  *       .module('demoApp', ['ngMaterial'])
@@ -64,11 +64,9 @@ angular
  *         });
  *   }
  *
- *   function DialogController(MdPanelRef, toppings) {
- *     var toppings;
- *
+ *   function DialogController(MdPanelRef) {
  *     function closeDialog() {
- *       MdPanelRef && MdPanelRef.close();
+ *       if (MdPanelRef) MdPanelRef.close();
  *     }
  *   }
  * })(angular);
@@ -526,11 +524,11 @@ angular
  * chosen; the last position will be chose whether it is on-screen or not.
  *
  * xPosition must be one of the following values available on
- * $mdPanel.xPosition:
+ * $mdPanel.xPosition:
  *
  *
  * CENTER | ALIGN_START | ALIGN_END | OFFSET_START | OFFSET_END
- *
+ *
  * <pre>
  *    *************
  *    *           *
@@ -543,14 +541,14 @@ angular
  * B: ALIGN_START (for LTR displays)
  * C: CENTER
  * D: ALIGN_END (for LTR displays)
- * E: OFFSET_END (for LTR displays)
+ * E: OFFSET_END (for LTR displays)
  * </pre>
  *
  * yPosition must be one of the following values available on
  * $mdPanel.yPosition:
  *
  * CENTER | ALIGN_TOPS | ALIGN_BOTTOMS | ABOVE | BELOW
- *
+ *
  * <pre>
  *   F
  *   G *************
@@ -564,7 +562,7 @@ angular
  * G: ALIGN_TOPS
  * H: CENTER
  * I: ALIGN_BOTTOMS
- * J: ABOVE
+ * J: ABOVE
  * </pre>
  *
  * @param {string} xPosition