Update offchain-storage.mdx

Fix typo: Line 156
`from: fromOption` --> `from: playerOption`

docs/zkapps/writing-a-zkapp/feature-overview/offchain-storage.mdx

@@ -153,7 +153,7 @@ class MyContract extends SmartContract {
 
     // increment the player's score, set a precondition on the previous score
     offchainState.fields.players.update(playerA, {
-      from: fromOption,
+      from: playerOption,
       to: score.add(1),
     });
   }