nucleotide-count: Remove mention of udacil

Focus the exercise on DNA and its nucleotides A, C, G and T.
The RNA-nucleotide U(dacil) shouldn't be treated specially.

As discussed in https://github.com/exercism/xpython/pull/98.
Merge pull request #18 from sjakobi/patch-2

nucleotide-count: Remove mention of udacil