doc: Fix unterminated inline literal in ceph-conf.rst

doc: Fix unterminated inline literal in ceph-conf.rst

Add missing space after closing backticks to properly terminate inline literal.

Sphinx was generating a warning due to an unterminated inline literal
string:

```
/home/kefu/dev/ceph/doc/rados/configuration/ceph-conf.rst:320: WARNING: Inline literal start-string without end-string.
```

This fix ensures the inline literal is properly closed and eliminates the
build warning.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>