Fix a typo made comparing previous prop (#7628)

The prop `prevIsAccountMenuOpen` was referenced in `prevProps`, despite
it not existing. It seems clear from the context that the intention
was to check the `isAccountMenuOpen` prop from `prevProps`, and name
the local variable `prevIsAccountMenuOpen`.

ui/app/components/app/account-menu/account-menu.component.js

@@ -28,7 +28,6 @@ export default class AccountMenu extends PureComponent {
     history: PropTypes.object,
     identities: PropTypes.object,
     isAccountMenuOpen: PropTypes.bool,
-    prevIsAccountMenuOpen: PropTypes.bool,
     keyrings: PropTypes.array,
     lockMetamask: PropTypes.func,
     selectedAddress: PropTypes.string,
@@ -42,7 +41,7 @@ export default class AccountMenu extends PureComponent {
   }
 
   componentDidUpdate (prevProps) {
-    const { prevIsAccountMenuOpen } = prevProps
+    const { isAccountMenuOpen: prevIsAccountMenuOpen } = prevProps
     const { isAccountMenuOpen } = this.props
 
     if (!prevIsAccountMenuOpen && isAccountMenuOpen) {