Hello,

I noticed the documentation specify that the `coverage-data->lcov`
function should be able to take modules to limit the scope of the data
that's coverted to lcov. From the docs:

 -- Scheme Procedure: coverage-data->lcov data port #:key modules
     Traverse code coverage information DATA, as obtained with
     ‘with-code-coverage’, and write coverage information to port in the
     ‘.info’ format used by LCOV
     (http://ltp.sourceforge.net/coverage/lcov.php).  The report will
     include all of MODULES (or, by default, all the currently loaded
     modules) even if their code was not executed.

However I noticed that this didn't match the code. I've attached a patch
which implements the behavour described in the docs.

Thanks,
Jessica.