tools: ceph-release-notes: escape _ for unintended links

tools: ceph-release-notes: escape _ for unintended links

foobar_ is taken as a hyper link by RST. so escape the underscores.
this change can avoid the warnings like:

ceph/doc/release-notes.rst:4593: ERROR: Unknown target name: "rgwgetusage".

where the source code looks like:
```
 rgw: need to 'open_object_section' before dump stats in 'RGWGetUsage_…(`issue#17499 <http://tracker.ceph.com/issues/17499>`_, `pr#11325 <http://github.com/ceph/ceph/pull/11325>\
`_, weiqiaomiao)
```
and apparently, "RGWGetUsage_" is not a hyper link here.

Signed-off-by: Kefu Chai <kchai@redhat.com>