docs: in doc comments, replace [aA]ngular2 with Angular (#15463)

packages/compiler-cli/src/extract_i18n.ts

@@ -11,7 +11,7 @@
 /**
  * Extract i18n messages from source code
  */
-// Must be imported first, because angular2 decorators throws on load.
+// Must be imported first, because Angular decorators throw on load.
 import 'reflect-metadata';
 
 import * as tsc from '@angular/tsc-wrapped';