Added the missing versionadded directive

Author: javiereguiluz

components/string.rst

@@ -512,6 +512,10 @@ the injected slugger is the same as the request locale::
 Inflector
 ---------
 
+..versionadded:: 5.1
+
+    The inflector feature was introduced in Symfony 5.1.
+
 In some scenarios such as code generation and code introspection, you need to
 convert words from/to singular/plural. For example, to know the property
 associated with an *adder* method, you must convert from plural