message: docs(lettable-operators): add missing quote
diff: @@ -24,7 +24,7 @@ because an operator returns an `Observable`,
  not a `Promise`.
  There is now an `Observable.toPromise()`instance method.
  -Because `throw` is a key word you could use `_throw` after `import { _throw } from 'rxjs/observable/throw`.
  +Because `throw` is a key word you could use `_throw` after `import { _throw } from 'rxjs/observable/throw'`.
  If the leading `_` bothers you (because a leading `_` typically means "_Internal - Do not use_"), you can do as follows:
  ```
  