Reword krb5_unparse_name_ext doxygen markup

Avoid using asterix characters in the documentation for
krb5_unparse_ext_name, since they get intepreted as markdown
punctuation when translated to RST.

https://github.com/krb5/krb5/commit/8ca5a84d3af27283ca6ef309e9e634bfb4ce52da
Author: Greg Hudson <ghudson@mit.edu>
Commit: 8ca5a84d3af27283ca6ef309e9e634bfb4ce52da
Branch: master
 src/include/krb5/krb5.hin |   11 ++++++-----
 1 files changed, 6 insertions(+), 5 deletions(-)