Formatting bug when rendering raw ASDF

[eslavich]

When an ASDF file's content is injected into documentation like this:

.. asdf:: test.asdf

The resulting rendered HTML looks like this:

test.asdfb'#ASDF' 1.5.0
%YAML 1.1
%TAG ! tag:stsci.edu:asdf/
# etc

Something is going on in that first line, the single quotes aren't actually part of the content and the filename shouldn't be run-on with the ASDF header.