doc: clean up manpage description of "notmuch-config list" output

doc: clean up manpage description of "notmuch-config list" output

The escaping in the description of the output of "notmuch-config list"
appears to have been inherited from some previous attempts at
documentation.  It leaked out in the actual generated manpage
documentation, where it looks like this:

       list   Every  configuration  item is printed to stdout, each on a
              separate line of the form:

                 *section*.\ *item*\ =\ *value*

This simplification cleans up the overescaping.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>