doc: fix troff warning in libgrokj2k.3 man page

Fix the following warning in libgrokj2k.3 man page:

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$ LC_ALL=it_IT.UTF-8 MANROFFSEQ='' MANWIDTH=80 \
  man --warnings -E UTF-8 -l -Tutf8 -Z doc/man/man3/libgrokj2k.3 >/dev/null
troff: <standard input>:127: warning: macro 'p' not defined
-----------------------------------------------------------------------

The issue was pointed out by the 'lintian' tool from Debian:

W: libgrokj2k1-dev: manpage-has-errors-from-man usr/share/man/man3/libgrokj2k.3.gz 127: warning: macro 'p' not defined