Fix DOI as requested by Kurt Hornik

[mrustl]

Dear maintainers,

This concerns the CRAN packages

kwb.hantush

maintained by one of you:

Michael Rustler michael.rustler@kompetenz-wasser.de: kwb.hantush

These have CRAN incoming check notes about DOIs in the package
description written as doi.org URLs instead of the actual DOIs.

Please change these to the form suggested by the notes (see below), and
also fix all other problems shown below and on your package check page.

Please fix before 2019-09-29 to safely retain your package on CRAN.
$kwb.hantush
Maintainer: ‘Michael Rustler michael.rustler@kompetenz-wasser.de’

The Description field contains
the Hantush (1967) equation (http://doi.org/10.1029/WR003i001p00227).
http://pubs.usgs.gov/sir/2010/5102/support/sir2010-5102.pdf).
Please enclose URLs in angle brackets (<...>).
The Description field contains
the Hantush (1967) equation (http://doi.org/10.1029/WR003i001p00227).
Please write DOIs as doi:10.prefix/suffix.