minor #9701 Fix factory-service new syntax (lex111)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #9701).

Discussion
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Fix factory-service new syntax

As I understand, this applies to all versions, not just 4.0. At least, the factory does not work for me, if I use only one `:`

https://github.com/symfony/dependency-injection/blob/e237279af4735c1566a62853c65de12b3266f0ef/Definition.php#L106-L108

Commits
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475cdc7 Fix factory-service new syntax

Author: javiereguiluz

service_container/factories.rst

@@ -106,7 +106,7 @@ Configuration of the service container then looks like this:
             app.newsletter_manager:
                 class:   AppBundle\Email\NewsletterManager
                 # call a method on the specified factory service
-                factory: 'app.newsletter_manager_factory:createNewsletterManager'
+                factory: 'app.newsletter_manager_factory::createNewsletterManager'
 
     .. code-block:: xml
 
@@ -160,7 +160,7 @@ Configuration of the service container then looks like this:
 
         app.newsletter_manager:
             # new syntax
-            factory: 'app.newsletter_manager_factory:createNewsletterManager'
+            factory: 'app.newsletter_manager_factory::createNewsletterManager'
             # old syntax
             factory: ['@app.newsletter_manager_factory', createNewsletterManager]