ofproto-dpif-trace: Remove tabs from output.

ofproto-dpif-trace: Remove tabs from output.

OVS uses spaces for indentation in source code and it makes sense for it to
also use spaces for indentation in output.  Spaces also consume less
horizontal space in output, which often makes it easier to read.  This
commit transitions one part of output from tabs to spaces and updates
appropriate parts of the tests to match.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>