doc/cephfs: Typos and small improvements in scrub.rst

doc/cephfs: Typos and small improvements in scrub.rst

Don't use space in "subdirectory" or "kick-start".

Change command syntax introduction text to the common "To ..., run a
command of the following form:" when the syntax has placeholders.

Use double backticks consistently (same as sections in the beginning)
for CLI commands, parameters etc.

Linkify "cluster health warnings" to the CephFS health warnings doc.

Use admonition and fix a typo in article.

Use title case for section titles and trim overly long underline.

Minor improvements to language.

Don't use unordered lists in two cases when there's no real need,
especially when there is only a single item in the latter case.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>