Improve command help page

Change-Id: I18ca3e222e472b21f699ee0dd3c5c8b4def00d61
diff --git a/cmd/datok.go b/cmd/datok.go
index 08b4160..bd797bf 100644
--- a/cmd/datok.go
+++ b/cmd/datok.go
@@ -18,11 +18,11 @@
 	} `kong:"cmd, help='Convert a foma file to a Matrix or Double Array tokenizer'"`
 	Tokenize struct {
 		Tokenizer         string `kong:"required,short='t',help='The Matrix or Double Array Tokenizer file'"`
-		Tokens            bool   `kong:"optional,negatable,default=true,help='Print token surfaces'"`
-		Sentences         bool   `kong:"optional,negatable,default=true,help='Print sentence boundaries'"`
-		TokenPositions    bool   `kong:"optional,negatable,default=false,short='p',help='Print token offsets'"`
-		SentencePositions bool   `kong:"optional,negatable,default=false,help='Print sentence offsets'"`
-		NewlineAfterEOT   bool   `kong:"optional,negatable,default=false,help='Ignore newline after EOT'"`
+		Tokens            bool   `kong:"optional,negatable,default=true,help='Print token surfaces (defaults to ${default})'"`
+		Sentences         bool   `kong:"optional,negatable,default=true,help='Print sentence boundaries (defaults to ${default})'"`
+		TokenPositions    bool   `kong:"optional,default=false,short='p',help='Print token offsets (defaults to ${default})'"`
+		SentencePositions bool   `kong:"optional,default=false,help='Print sentence offsets (defaults to ${default})'"`
+		NewlineAfterEOT   bool   `kong:"optional,default=false,help='Ignore newline after EOT (defaults to ${default})'"`
 	} `kong:"cmd, help='Tokenize a text'"`
 }