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.

```
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
```