free: apply conventions

- Used the multiple options convention properly
- Added a concrete example

pages/linux/free.md

@@ -8,12 +8,12 @@
 
 - Display memory in Bytes/KB/MB/GB:
 
-`free {{-b/-k/-m/-g}}`
+`free -{{b|k|m|g}}`
 
 - Display memory in human readable units:
 
 `free -h`
 
-- Continuous monitor memory (refresh every X seconds):
+- Refresh the output every 2 seconds:
 
-`free -s {{X}}`
+`free -s {{2}}`