docs: fix typo

Co-authored-by: Joseph Livesey <[email protected]>

Author: gusinacio

tap_eip712_message/src/lib.rs

@@ -127,7 +127,7 @@ impl<M: SolStruct> EIP712SignedMessage<M> {
         }
     }
 
-    /// Use this a simple key for testing
+    /// Use this as a simple key for testing
     pub fn unique_hash(&self) -> MessageId {
         MessageId(self.message.eip712_hash_struct().into())
     }