Fix typo in $Call documentation

See piotrwitek#149 . I don't know if this patch should close that issue, though.

README.md

@@ -1066,7 +1066,7 @@ import { $Call } from 'utility-types';
 
 // Common use-case
 const add = (amount: number) => ({ type: 'ADD' as 'ADD', payload: amount });
-type AddAction = $Call<typeof returnOfIncrement>; // { type: 'ADD'; payload: number }
+type AddAction = $Call<typeof add>; // { type: 'ADD'; payload: number }
 
 // Examples migrated from Flow docs
 type ExtractPropType<T extends { prop: any }> = (arg: T) => T['prop'];