mf6io.pdf - LAK: CTABNAME in LAK not defined

[emorway-usgs]

Moved this comment to modflow6 repo from https://github.com/OGW-USGS/modflow6-docs/issues/4

The definition for tab6_filename includes a reference to CTABNAME that isn't defined in the mf6io documentation. Not knowing all that is going on under the hood with the tex files, I wonder if the first instance of CTABNAME should instead be something like "The file tab6_filename includes the number of entries..."?

[jdhughes-usgs]

@langevin-usgs if you are ok with pull request we should get this in the release after Richard's review.