docs(auth-guard): typo (#2170)

Remove duplicate build wording

Author: ChesterCorin

docs/auth/router-guards.md

@@ -61,7 +61,7 @@ export const routes: Routes = [
 
 ### Compose your own pipes
 
-`AngularFireAuthGuard` pipes are RXJS operators which transform an optional User to a boolean or Array (for redirects). You can build easily build your own to customize behavior further:
+`AngularFireAuthGuard` pipes are RXJS operators which transform an optional User to a boolean or Array (for redirects). You can easily build your own to customize behavior further:
 
 ```ts
 import { map } from 'rxjs/operators';
@@ -99,4 +99,4 @@ const onlyAllowSelf = (next) => map(user => !!user && next.params.userId === use
 // Only allow navigation to the route if the user has a custom claim matching  :accountId
 // { path: 'accounts/:accountId/billing', component: BillingDetailsComponent, ...canActivate(accountAdmin) }
 const accountAdmin = (next) => pipe(customClaims, map(claims => claims[`account-${next.params.accountId}-role`] === "admin"));
-```
+```