cal: Add use case

For clarity I changed "-m" to "--monday".
Also, I've removed the "cal -y" for simplicity.
You could just user "cal {year}" to get the same information.

pages/linux/cal.md

@@ -6,13 +6,13 @@
 
 `cal`
 
+- Display previous, current and next month: 
+
+`cal --three`
+
 - Use monday as the first day of the week:
 
-`cal -m`
-
-- Display a calendar for the current year:
-
-`cal -y`
+`cal --monday`
 
 - Display a calendar for a specific year (4 digits):