message: Fixes typo in operators.md file
  Changes the `Observalbes` to `Observable` in Higher-order Observables section.
diff: @@ -82,7 +82,7 @@ const fileObservable = urlObservable.pipe(
  );
  ```
  -`http.get()` returns an Observable (of string or string arrays probably) for each individual URL. Now you have an Observables *of* Observables, a higher-order Observable.
  +`http.get()` returns an Observable (of string or string arrays probably) for each individual URL. Now you have an Observable *of* Observables, a higher-order Observable.
  But how do you work with a higher-order Observable? Typically, by _flattening_: by (somehow) converting a higher-order Observable into an ordinary Observable. For example:
  