minor #15137 [DependencyInjection] Fixed invalid alias definition in "Deprecating Service Aliases" YAML example (adamwojs)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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[DependencyInjection] Fixed invalid alias definition in "Deprecating Service Aliases" YAML example

Fixed invalid alias definition in "Deprecating Service Aliases" YAML example. Current version is

```yaml
app.mailer:
    alias: '@app\Mail\PhpMailer'

    # ....
```
Correct version is

```yaml
app.mailer:
    alias: 'App\Mail\PhpMailer'

    # ....
```

(without `@` prefix in service id)

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Commits
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3938dbd Fixed invalid alias definition in "Deprecating Service Aliases" YAML example

Author: javiereguiluz

service_container/alias_private.rst

@@ -166,7 +166,7 @@ or you decided not to maintain it anymore), you can deprecate its definition:
     .. code-block:: yaml
 
         app.mailer:
-            alias: '@App\Mail\PhpMailer'
+            alias: 'App\Mail\PhpMailer'
 
             # this will display a generic deprecation message...
             deprecated: true