Fix description of mt/n ratio in docs

[TCLamnidis]

Currently, the eager documentation defines Mt/nuc ratio as

MT to Nuclear Ratio This from MTtoNucRatio. This reports the number of reads aligned to a mitochondrial entry in your reference FASTA to all other entries. This will typically be high but will vary depending on tissue type.

This is incorrect, as the ratio is actually between the mean coverages between the two reference parts.
It is not worthwhile to make a new patch for such a small change to the docs, but we should make sure the wording gets corrected for eager 3.*.