tail: harmonize description placeholders with code (#18220)

Author: ivanbaluta

pages/common/tail.md

@@ -24,6 +24,6 @@
 
 `tail {{[-F|--retry --follow]}} {{path/to/file}}`
 
-- Show last 'num' lines in 'file' and refresh every 'n' seconds:
+- Show last 'count' lines in 'file' and refresh every 'seconds' seconds:
 
 `tail {{[-n|--lines]}} {{count}} {{[-s|--sleep-interval]}} {{seconds}} {{[-f|--follow]}} {{path/to/file}}`