Fix typo in spec.md

Signed-off-by: Daisuke Gunji <[email protected]>

Author: GunjiD

content/docs/spec.md

@@ -473,7 +473,7 @@ Plugins can return one of three result types:
 
 - _Success_ (or _Abbreviated Success_)
 - _Error_
-- _Version
+- _Version_
 
 ### Success