README: fix statement about test file locations

Created by: bennofs

hs-source-dirs may contain multiple directories, which are not subdirectories of each other. For example, the following is perfectly fine:

hs-source-dirs: tests/ anotherTestDir/

tasty-discover will not find tests in anotherTestDir (if Tasty.hs is in tests/Tasty.hs), so the previous statement about the location of tests was wrong.