chore(docs): updated instructions for angular-seed (#4252)

* Update NPM dependencies

Bootstrap v4 removed bootstrap-theme

* Sync with upstream: s/let/const/

docs/getting-started/angular-seed.md

@@ -17,13 +17,14 @@ this.NPM_DEPENDENCIES = [
   // {src: 'lodash/lodash.min.js', inject: 'libs'},
   { src: 'bootstrap/dist/js/bootstrap.min.js', inject: 'libs' },
   { src: 'bootstrap/dist/css/bootstrap.min.css', inject: true }, // inject into css section
-  { src: 'bootstrap/dist/css/bootstrap-theme.min.css', inject: true }, // inject into css section
-  { src: 'bootstrap/dist/css/bootstrap-theme.min.css.map', inject: true }, // inject into css section
+  // Uncomment when using Bootstrap v3. Bootstrap v4 removed bootstrap-theme
+  // { src: 'bootstrap/dist/css/bootstrap-theme.min.css', inject: true }, // inject into css section
+  // { src: 'bootstrap/dist/css/bootstrap-theme.min.css.map', inject: true }, // inject into css section
 ];
 
 // *towards the bottom, replace extended packages with this:
 
-let additionalPackages: ExtendPackages[] = [
+const additionalPackages: ExtendPackages[] = [
 // required for dev build
 {
   name: 'ngx-bootstrap',