bug symfony#4637 fixed StringExpressionLanguageProvider code example symfony#4636 (danieleorler)

This PR was squashed before being merged into the 2.6 branch (closes symfony#4637).

Discussion
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fixed StringExpressionLanguageProvider code example symfony#4636

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6
| Fixed tickets | symfony#4636

Commits
-------

33e0a7f fixed StringExpressionLanguageProvider code example symfony#4636

Author: wouterj

components/expression_language/extending.rst

@@ -77,17 +77,20 @@ register.
 
     class StringExpressionLanguageProvider implements ExpressionFunctionProviderInterface
     {
-        return array(
-            new ExpressionFunction('lowercase', function ($str) {
-                return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
-            }, function ($arguments, $str) {
-                if (!is_string($str)) {
-                    return $str;
-                }
-
-                return strtolower($str);
-            });
-        );
+        public function getFunctions()
+        {
+            return array(
+                new ExpressionFunction('lowercase', function ($str) {
+                    return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
+                }, function ($arguments, $str) {
+                    if (!is_string($str)) {
+                        return $str;
+                    }
+
+                    return strtolower($str);
+                }),
+            );
+        }
     }
 
 You can register providers using