mgr/dashboard: Fix inline markup warning in API documentation

mgr/dashboard: Fix inline markup warning in API documentation

Remove trailing space from summary field that was causing Sphinx build
warning.

Sphinx was generating a warning due to malformed inline markup:

```
/home/kefu/dev/ceph/doc/mgr/ceph_api/index.rst:3349: WARNING: Inline strong start-string without end-string.`
```

The openapi directive appears to convert trailing spaces into asterisk
markers, creating unterminated strong markup. This change removes the
trailing space to eliminate the warning and maintain consistency with
other entries in the file.

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