fix(docs): wrong payment reference length in fsa overview

Author: nikerzetic-aflabs

docs/smart-accounts/1-overview.mdx

@@ -44,7 +44,7 @@ The payment receipt is a `bytes32` value.
 The first byte is reserved for the instruction code.
 The second byte is a wallet identifier;
 this is a number assigned to wallet providers by the Flare Foundation, and should otherwise be `0`.
-The remaining 31 bytes are the parameters for the chosen instruction.
+The remaining 30 bytes are the parameters for the chosen instruction.
 
 In practice, the payment receipt should be prepared by a backend, through a web form.