Update doc/source/command-objects/image.rst

Update doc/source/command-objects/image.rst to match
output of 'openstack help image create' again.

Forgot this in:
https://review.openstack.org/#/c/437335/

Change-Id: Ied7ed88ea79da0b778cccf19d087b5ee06edcb71

Author: DavidRabel-b1

doc/source/command-objects/image.rst

@@ -53,8 +53,7 @@ Create/upload an image
         [--min-ram <ram-mb>]
         [--location <image-url>]
         [--copy-from <image-url>]
-        [--file <file>]
-        [--volume <volume>]
+        [--file <file> | --volume <volume>]
         [--force]
         [--checksum <checksum>]
         [--protected | --unprotected]