[cli] Fix typo in verify-json help message.

src/main.ml

@@ -320,7 +320,7 @@ let verify_json_cmd =
   let cmdname = "verify-json" in
   let info =
     let doc =
-      "Property verification using external provers via json configuration \
+      "Property verification using external provers via a JSON configuration \
        file."
     in
     Cmd.info cmdname ~sdocs:Manpage.s_common_options ~exits:Cmd.Exit.defaults