docs(panel): fix ampersands getting parsed into & (#11450)

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## PR Checklist
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- [x] The commit message follows [our guidelines](https://github.com/angular/material/blob/master/.github/CONTRIBUTING.md#-commit-message-format)
- [x] Tests for the changes have been added or this is not a bug fix / enhancement
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## PR Type
What kind of change does this PR introduce?
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[ ] Bugfix
[ ] Enhancement
[x] Documentation content changes
[ ] Code style update (formatting, local variables)
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## What is the current behavior?
Some of the code in the example was getting parsed to `&amp;&amp;`.
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Issue Number: 
N/A

## What is the new behavior?
Fixed to not use `&&` in this example.

## Does this PR introduce a breaking change?
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[ ] Yes
[x] No
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## Other information

Author: Splaktar

src/components/panel/panel.js

@@ -111,9 +111,11 @@ angular
  *   }
  *
  *   function PanelMenuCtrl(mdPanelRef) {
- *     // The controller is provided with an import named 'mdPanelRef'
+ *     // 'mdPanelRef' is injected in the controller.
  *     this.closeMenu = function() {
- *       mdPanelRef && mdPanelRef.close();
+ *       if (mdPanelRef) {
+ *         mdPanelRef.close();
+ *       }
  *     };
  *   }
  * })(angular);