Use Pandoc also for plain-text documentation output

The makefile rule for the (rarely used) plain-text output postgres.txt
was still written to use lynx, but in
96b8b8b6f9, where the INSTALL file was
switched to pandoc, the rest of the makefile support for lynx was
removed, so this was broken.  Rewrite the rule to also use pandoc for
postgres.txt.

doc/src/sgml/Makefile

@@ -142,7 +142,7 @@ postgres.html: stylesheet-html-nochunk.xsl postgres.sgml $(ALLSGML)
 
 # single-page text
 postgres.txt: postgres.html
-	$(LYNX) -force_html -dump -nolist $< > $@
+	$(PANDOC) -t plain -o $@ $<
 
 
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