common/options: fix invalid command for STS key generation
Config rgw_sts_key expects a 16-character hexadecimal key. The option
long_desc has an example with openssl rand -hex 16 which generates
16 hexadecimal bytes that consist of 32 characters rendered in ASCII.
Modify the example openssl command to output 8 bytes that matches the
required length of 16 characters.
Remove a space after a slash.
Raised in the doc bugs pad.
Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>