docs: fix README code on decoding tx instruction

[GCrispino]

Fixes issues on code sample provided in section "Parse/decode an instruction from a transaction" from the README:

• if if typo replaced with if;
• Error check added to call to function ResolveInstructionAccounts;
• Example transaction fetched was changed because the transaction hash that was there could not be found by the solana public testnet RPC endpoint.

[gagliardetto]

Thanks!

You're right, I should have probably selected a transaction from mainnet because the other clusters do get purged from time to time.

[GCrispino]

👍🏼 👍🏼

Let me know if you have any suggestion to add to the PR

[gagliardetto]

Thanks!

[GCrispino]

Thanks!

Thanks for merging!