Update constant_attrib.md

Author: HMohammad2520

docs/constant_attrib.md

@@ -31,6 +31,8 @@ This is useful when you want:
 ## Basic Usage
 
 ```python
+from classmods import ConstantAttrib
+
 class MyClass:
     VALUE = ConstantAttrib()
 
@@ -148,4 +150,4 @@ Recommended for:
 Avoid using when:
 
 * Values must change
-* Full immutability is required (use frozen dataclasses instead)
+* Full immutability is required (use frozen dataclasses instead)