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

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

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/Makefile

@@ -124,7 +124,7 @@ endif
 
 html/%.txt: %.txt
 	$(INSTALL_DIR) $(@D)
-	cp $< $@
+	$(INSTALL_DATA) $< $@
 
 html/man/%.1.html: man/%.pod.1 Makefile
 	$(INSTALL_DIR) $(@D)