chore(icon): fix comment referring to deprecated Md (#8940)

Author: andrewseguin

src/lib/icon/icon-registry.ts

@@ -488,7 +488,7 @@ export function ICON_REGISTRY_PROVIDER_FACTORY(
 
 /** @docs-private */
 export const ICON_REGISTRY_PROVIDER = {
-  // If there is already an MdIconRegistry available, use that. Otherwise, provide a new one.
+  // If there is already an MatIconRegistry available, use that. Otherwise, provide a new one.
   provide: MatIconRegistry,
   deps: [
     [new Optional(), new SkipSelf(), MatIconRegistry],