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>