fix(material/dialog): dialog name not read by mac screenreader on chrome and firefox

Fixes Angular Components Dialog component where the mac screenreader is not
reading the associated aria-labelledby id value as the dialog name/title.
Fixes logic checking for macos so that the function_getDialogName() gets
called when the value of the arialabelledby id element gets inserted into
the aria-label value.

Fixes b/274674581

Author: essjay05

src/material/dialog/dialog-container.ts

@@ -178,7 +178,7 @@ export class MatDialogContainer
 
   private _setAriaLabel = (): void => {
     const os = this._getUserPlatform();
-    if (os === 'mac') {
+    if (os === 'macos') {
       this._getDialogName();
     }
     return;