nit(combobox): changed double quotes to single quotes.

Author: nielsr98

src/cdk-experimental/combobox/combobox-module.ts

@@ -10,7 +10,7 @@ import {NgModule} from '@angular/core';
 import {OverlayModule} from '@angular/cdk/overlay';
 import {CdkCombobox} from './combobox';
 import {CdkComboboxPanel} from './combobox-panel';
-import {CdkComboboxPopup} from "./combobox-popup";
+import {CdkComboboxPopup} from './combobox-popup';
 
 const EXPORTED_DECLARATIONS = [CdkCombobox, CdkComboboxPanel, CdkComboboxPopup];
 @NgModule({