tail: harmonize description placeholders with code

[ivanbaluta]

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

This PR fixes a minor inconsistency in the description for the last example on the tail page.
The description used the placeholders num, file, and n, while the code example below uses count, path/to/file, and seconds.
This change harmonizes the description with the code, making the page clearer and easier to translate accurately.

[Managor]

Good catch. This inconsistency was introduced in #7681