src/common: Fix formatting in options/rgw.yaml.in

src/common: Fix formatting in options/rgw.yaml.in

Fix single backticks to double backticks to properly end the inline
preformatted formatting. Fixes the formatting overflowing until the next
occurrence of double backticks seen in rendered docs, URL:
https://docs.ceph.com/en/latest/radosgw/config-ref/#confval-rgw_scheduler_type

Add full stops that seemed to be missing in desc attribute.
Use singular word "value" in desc attribute when there's only one
possible other value.

Remove unnecessary "the".

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