docs: improve comment consistency in .muttrc example configuration

Added a space to align comment formatting; this helps improve
consistency and visual uniformity.

Signed-off-by: Jiamu Sun <barroit@linux.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/SY0P300MB0801D1A4B278157CA7C92DE2CEBC2@SY0P300MB0801.AUSP300.PROD.OUTLOOK.COM