minor #21265 [ExpressionLanguage] Fix backslashes count (MatTheCat)

This PR was merged into the 6.4 branch.

Discussion
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[ExpressionLanguage] Fix backslashes count

Since a backslash must be escaped by three backslashes, you need four in total.

Commits
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86a10b4 [ExpressionLanguage] Fix backslashes count

Author: javiereguiluz

reference/formats/expression_language.rst

@@ -178,7 +178,7 @@ This also works with class constants::
     }
 
     var_dump($expressionLanguage->evaluate(
-        'constant("App\\\SomeNamespace\\\Foo::API_ENDPOINT")'
+        'constant("App\\\\SomeNamespace\\\\Foo::API_ENDPOINT")'
     ));
 
 This will print out ``/api``.
@@ -196,7 +196,7 @@ This function will return the case of an enumeration::
     }
 
     var_dump(App\Enum\Foo::Bar === $expressionLanguage->evaluate(
-        'enum("App\\\SomeNamespace\\\Foo::Bar")'
+        'enum("App\\\\SomeNamespace\\\\Foo::Bar")'
     ));
 
 This will print out ``true``.