Update observer.md (#6387)

* Update observer.md

migrate to 7.

* Update observer.md

* docs: docs_app/content/guide/observer.md

* docs: docs_app/content/guide/observer.md

Co-authored-by: Nicholas Jamieson <nicholas@cartant.com>

docs_app/content/guide/observer.md

@@ -20,7 +20,7 @@ observable.subscribe(observer);
 
 Observers in RxJS may also be *partial*. If you don't provide one of the callbacks, the execution of the Observable will still happen normally, except some types of notifications will be ignored, because they don't have a corresponding callback in the Observer.
 
-The example below is an Observer without the `complete` callback:
+The example below is an `Observer` without the `complete` callback:
 
 ```ts
 const observer = {
@@ -29,18 +29,10 @@ const observer = {
 };
 ```
 
-When subscribing to an Observable, you may also just provide the callbacks as arguments, without being attached to an Observer object, for instance like this:
+When subscribing to an `Observable`, you may also just provide the next callback as an argument, without being attached to an `Observer` object, for instance like this:
 
 ```ts
 observable.subscribe(x => console.log('Observer got a next value: ' + x));
 ```
 
-Internally in `observable.subscribe`, it will create an Observer object using the first callback argument as the `next` handler. All three types of callbacks may be provided as arguments:
-
-```ts
-observable.subscribe(
-  x => console.log('Observer got a next value: ' + x),
-  err => console.error('Observer got an error: ' + err),
-  () => console.log('Observer got a complete notification')
-);
-```
+Internally in `observable.subscribe`, it will create an `Observer` object using the callback argument as the `next` handler.