doc: Fix minor formatting, typo etc issues

doc: Fix minor formatting, typo etc issues

Remove formatting syntax from inside literal text in
cephadm/services/rgw.rst.
Use quotation marks similarly to other placement examples with only
parameter value quoted and not the whole parameter in
cephadm/services/rgw.rst.
Capitalize "YAML" in cephadm/services/rgw.rst.

Remove double space in the middle of a sentence in
rados/operations/erasure-code.rst.

Use double backticks consistently for default values in
radosgw/frontends.rst.
Capitalize "I/O", stylize as "OpenSSL" in radosgw/frontends.rst.

Fix typo "and object" to "an object" in radosgw/s3/bucketops.rst.

Stylize as "CentOS" in start/os-recommendations.rst.

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