docs/tools/virtiofsd: Fix bad rst syntax

For literal blocks, there has to be an empty line after the two colons,
and the block itself should be indented.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210607180015.924571-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/tools/virtiofsd.rst

@@ -250,6 +250,7 @@ the host set.
 This is equivalent to the 'map' rule:
 
 ::
+
  -o xattrmap=":map::user.virtiofs.:"
 
 2) Prefix 'trusted.' attributes, allow others through
@@ -277,6 +278,7 @@ through.
 This is equivalent to the 'map' rule:
 
 ::
+
  -o xattrmap="/map/trusted./user.virtiofs./"
 
 3) Hide 'security.' attributes, and allow everything else