Documentation: Improve wording on the Variables section

Thanks @crewjam for the suggestion

Closes #234

Author: andreynering

docs/usage.md

@@ -400,7 +400,7 @@ tasks:
 When doing interpolation of variables, Task will look for the below.
 They are listed below in order of importance (e.g. most important first):
 
-- Variables declared locally in the task
+- Variables declared in the task definition
 - Variables given while calling a task from another.
   (See [Calling another task](#calling-another-task) above)
 - Variables declared in the `vars:` option in the `Taskfile`