fix(docs): correct validateSchema example

* Correct `validateSchema` example variable name

The function gets a `value` argument but the body was referring to an `age` variable (which wouldn't exist here). It should test the `value` parameter (alternatively, rename the `value` argument to `age`).

* Update README.md

README.md

@@ -175,8 +175,10 @@ compose(
       required: true
     },
     age: {
-      test: (value) => {
-        return age && age > 18
+      test: (value, fail) => {
+        if (!value || value <= 18) {
+          return fail('Age must be 18 or older');
+        }
       }
     }
   })