Fix dbtoepub output file name

In previous releases, the input file of dbtoepub was postgres.xml, and
dbtoepub knows to derive the output file name postgres.epub from that
automatically.  But now the intput file is postgres.sgml (since
postgres.sgml is itself an XML file and we no longer need the
intermediate postgres.xml file), but dbtoepub doesn't know how to deal
with the .sgml suffix, so the automatically derived output file name
becomes postgres.sgml.epub.  Fix by adding an explicit -o option.

doc/src/sgml/Makefile

@@ -171,7 +171,7 @@ postgres.pdf:
 epub: postgres.epub
 postgres.epub: postgres.sgml $(ALLSGML)
 	$(XMLLINT) --noout --valid $<
-	$(DBTOEPUB) $<
+	$(DBTOEPUB) -o $@ $<
 
 
 ##