Added the way to modify immutable string. (#25334)

* Added the way to modify immutable string.

* Formatting changes

guide/english/python/data-structures/strings/index.md

@@ -20,6 +20,14 @@ Python allows `str` objects, or _strings_, to be expressed in a few different wa
                 File "<stdin>", line 1, in <module>
         TypeError: 'str' object does not support item assignment
         
+    Instead, you can convert the string into a list, modify the list element (string character) you wish to change, and then join  the list elements back to a string, like so:
+        
+        >>> foo = "my string"
+        >>> foo_list_form = list(foo)
+        >>> foo_list_form[0] = "a"
+        >>> foo = ' '.join(foo_list_form)
+        >>> print(foo)
+        ay string # The required output
         
 ## Reference: