Fix incorrect and missing fret symbols.

[jamieforth]

No description provided.

[ironchicken]

Hi @jamieforth. This all looks good. I can't believe I fell into the trap of including 'J'!

One problem, however, is that now some of the tests fail. You can enable the test suite with:

$ cabal configure --enable-tests
$ cabal install --enable-tests --only-dependencies

And then run the tests with:

$ cabal test --show-details=always

The ones that are failing are in tests/ParseTests.hs:233 in the block of failChords. These are tests of chords that are invalid and the tests pass if the parser correctly identifies them as invalid. If you look at the test on line 241 you can see that it's saying that a chord with a note on fret o is invalid. Of course, this is no longer the case. We could fix these tests by replacing the os with js.