message: chore(find): fix compat signatures
diff: @@ -4,10 +4,10 @@ import { find as higherOrder } from 'rxjs/operators';
  /* tslint:disable:max-line-length */
  export function find<T, S extends T>(this: Observable<T>,
  predicate: (value: T, index: number) => value is S,
  - thisArg?: any): Observable<S>;
  + thisArg?: any): Observable<S | undefined>;
  export function find<T>(this: Observable<T>,
  predicate: (value: T, index: number) => boolean,
  - thisArg?: any): Observable<T>;
  + thisArg?: any): Observable<T | undefined>;
  /* tslint:enable:max-line-length */
  /**
  @@ -44,6 +44,6 @@ export function find<T>(this: Observable<T>,
  * @owner Observable
  */
  export function find<T>(this: Observable<T>, predicate: (value: T, index: number, source: Observable<T>) => boolean,
  - thisArg?: any): Observable<T> {
  + thisArg?: any): Observable<T | undefined> {
  return higherOrder(predicate, thisArg)(this);
  }
  