fix: add missing OP_CHECKMULTISIG to p2ms locking script

Author: GabrieleBocchi

decoding/p2ms.mdx

@@ -70,6 +70,7 @@ M
 <PubKey2>
 <PubKeyN>
 N
+OP_CHECKMULTISIG
   `}
     language="text"
     showLineNumbers={true}