Update attribute.md,

#![allow(unusude_variables)] -> #![allow(unused_variables)]

Author: culxttes

src/attribute.md

@@ -33,7 +33,7 @@ with the difference between them being where they apply.
 - `#![inner_attribute]` applies to the enclosing [item][item] (typically a
   module or a crate). In other words, this attribute is interpreted as
   applying to the entire scope in which it's place. Here is an example
-  where `#![allow(unusude_variables)]` applies to the whole crate (if
+  where `#![allow(unused_variables)]` applies to the whole crate (if
   placed in `main.rs`):
   ```rust
   #![allow(unused_variables)]