community/moreutils: fix docbook-xsl stylesheets path

_docbookpath contained multiple directory paths from a recent change to docbook-xsl subpackages. This led to an error with generating the man pages and caused some executables to not be output from the same make option. Since some executables could not be found during make install, the build failed during the package stage. Restoring a valid _docbookpath value enabled the man pages to be successfully generated.

Error found on the 3.21 builders (error log):

make: Nothing to be done for '/usr/share/xml/docbook/xsl-stylesheets-1.79.2-nons'.
make: Nothing to be done for '/usr/share/xml/docbook/xsl-stylesheets-nons'.
[...]
install -s isutf8 ifdata ifne pee sponge mispipe lckdo parallel errno
/home/buildozer/aports/community/moreutils/pkg/moreutils/usr/bin
install: can't stat 'ifdata': No such file or directory
install: can't stat 'ifne': No such file or directory
install: can't stat 'pee': No such file or directory
install: can't stat 'sponge': No such file or directory
install: can't stat 'mispipe': No such file or directory
install: can't stat 'lckdo': No such file or directory
install: can't stat 'parallel': No such file or directory
install: can't stat 'errno': No such file or directory
make: *** [Makefile:23: install] Error 1