Remove non-existing increment and decrement export (#21)

Co-authored-by: Emili Pallares <[email protected]>

src/exercise/01.md

@@ -39,12 +39,12 @@ function useCounter() {
  return context
 }
 
-export {CounterProvider, useCounter, increment, decrement}
+export {CounterProvider, useCounter}
 ```
 
 ```javascript
 // src/screens/counter.js
-import {useCounter, increment, decrement} from 'context/counter'
+import {useCounter} from 'context/counter'
 
 function Counter() {
  const [state, dispatch] = useCounter()