doc: correct spelling of "fqdn"

The "fqdn" (fully-qualified domain name) was misspelled in a couple
places as fgdn". Use the correct spelling.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
doc: correct grammar in RGW SSL cert installation

The SSL certificate generation instruction was missing a preposition.
Add it.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Merge pull request #2901 from ktdreyer/doc-spelling-grammar

doc: spelling & grammar fixes

Reviewed-by: John Wilkins <jowilkin@redhat.com>