docs: admin-guide/sysctl: Fix rendering error

Text in ``literal`` markup must be separated by word separators, so text
like ``lowwater``% renders incorrectly.  Add the suggested "\ " after two
problematic occurrences.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20220624110230.595740-1-steve@sk2.org


[jc: tweaked to use "\ "]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>