Fix typo: Additionally

Co-authored-by: Aditya Sirish <[email protected]>

Author: MarkLodato

specification.md

@@ -237,7 +237,7 @@ The single-message data structure / file format:
 *   SHOULD be easy to parse using common libraries (e.g. JSON)
 *   SHOULD support a hint indicating what signing key was used
 
-Addtionally, a multi-message file format:
+Additionally, a multi-message file format:
 
 *   MUST support multiple signed messages in a single file, possibly as a
     separate file format