docs: make .txt files writable when building from r/o sources

Otherwise an incremental build will fail to overwrite the destination
files.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/docs/Makefile
+++ b/docs/Makefile
@@ -125,6 +125,7 @@ endif
 html/%.txt: %.txt
 	$(INSTALL_DIR) $(@D)
 	cp $< $@
+	chmod u+w $@
 
 html/man/%.1.html: man/%.pod.1 Makefile
 	$(INSTALL_DIR) $(@D)