minor fix (#7828)

substrate/primitives/externalities/src/extensions.rs

@@ -98,7 +98,7 @@ pub trait ExtensionStore {
 	/// instead of this function to get type system support and automatic type downcasting.
 	fn extension_by_type_id(&mut self, type_id: TypeId) -> Option<&mut dyn Any>;
 
-	/// Register extension `extension` with speciifed `type_id`.
+	/// Register extension `extension` with specified `type_id`.
 	///
 	/// It should return error if extension is already registered.
 	fn register_extension_with_type_id(&mut self, type_id: TypeId, extension: Box<dyn Extension>) -> Result<(), Error>;