[exim] FAQ Q0413 Update

http://www.exim.org/exim-html-4.40/doc/html/FAQ_4.html#TOC156

In the router, 'prefix' needs to be 'local_part_prefix' now with the
recent Exim. Found it while implementing the FAQ. ;)

-te

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Troy Engel | Systems Engineer
Fluid, Inc | http://www.fluid.com