doc: fix Unicode character for apostrophe

cd doc/pdf && pdflatex halmd.tex:

! Package inputenc Error: Unicode char \u8:ʹ not set up for use with LaTeX.

See the inputenc package documentation for explanation.
Type  H <return>  for immediate help.
 ...

l.8973 \emph{HALʹs MD package}
                                is maintained in a public Git repository. \h...