docs: Fix variable declaration for shouldError

Declaring the `shouldError` as `const` as it is not changed but only compared to ensure good practice.

Author: moijes12

src/content/learn/reacting-to-input-with-state.md

@@ -468,7 +468,7 @@ function submitForm(answer) {
   // Pretend it's hitting the network.
   return new Promise((resolve, reject) => {
     setTimeout(() => {
-      let shouldError = answer.toLowerCase() !== 'lima'
+      const shouldError = answer.toLowerCase() !== 'lima'
       if (shouldError) {
         reject(new Error('Good guess but a wrong answer. Try again!'));
       } else {