man/fi_getinfo.3: Clean up return values section

Fix typo in description of fi_dupinfo() error return.
Clarify that first paragraph refers to fi_getinfo().

Fixes #259

Signed-off-by: Patrick MacArthur <pmacarth@iol.unh.edu>
Merge pull request #309 from patrickmacarthur/dupinfo-man-fix

man/fi_getinfo.3: Clean up return values section