minor #12504 Escape backslash before string termination (greg0ire)

This PR was submitted for the master branch but it was merged into the 4.3 branch instead (closes #12504).

Discussion
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Escape backslash before string termination

To php, `'App\'` looks like a string that contains a quote and is not finished. Using `\\`, the only escape sequence that works within single quotes, should fix the issue.
This was introduced in 2a5b114 , which is only present on the master branch at the time, that's why I'm targetting master.

Commits
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e92115e Escape backslash before string termination

Author: javiereguiluz

service_container/configurators.rst

@@ -176,7 +176,7 @@ all the classes are already loaded as services. All you need to do is specify th
             $services = $configurator->services();
 
             // Registers all 4 classes as services, including App\Mail\EmailConfigurator
-            $services->load('App\', '../src/*');
+            $services->load('App\\', '../src/*');
 
             // override the services to set the configurator
             $services->set(NewsletterManager::class)
@@ -250,7 +250,7 @@ routes can reference :ref:`invokable controllers <controller-service-invoke>`.
             $services = $configurator->services();
 
             // Registers all 4 classes as services, including App\Mail\EmailConfigurator
-            $services->load('App\', '../src/*');
+            $services->load('App\\', '../src/*');
 
             // override the services to set the configurator
             $services->set(NewsletterManager::class)