truncate.1: improve the example section

Notably:

• spell "Megabytes" as "megabytes" consistently;
• remove a stray asterisk from the /boot/kernel/kernel listing
• avoid using a shell prompt in the examples consistently
• fixes the size parameter
• add an example how to increase the file size

PR: 273997
MFC after: 1 week

(cherry picked from commit 5460bdda9d4cb3ce1d7431824b6f53c545888b53)