Fix bug in api2text.pl stanza.  ../../../doc/docbook/src/plplotdoc.xml.in ==> ../../../doc/docbook/src/plplotdoc.xml.  The former file has a configurable DOCTYPE line which is only expressed in the correct form in the latter file.
Fix bug in api2text.pl stanza.  ../../../doc/docbook/src/plplotdoc.xml.in
==> ../../../doc/docbook/src/plplotdoc.xml.  The former file has a configurable
DOCTYPE line which is only expressed in the correct form in the latter file.

This change may solve Joao's recent problems parsing api.xml, and in fact I
am surprised this worked at all before for Debian systems since there was no
valid DOCTYPE.

svn path=/trunk/; revision=4551

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot