verify-grammar path to rust.md

When calling

   make verify-grammar

rust.md cannot be found, because the
path to rust.md is missing.

The path is set to:

   $(D)/rust.md

This can only be tested, when llnextgen is installed.

Signed-off-by: Jan Kobler <eng1@koblersystems.de>

mk/docs.mk

@@ -232,7 +232,7 @@ ifeq ($(CFG_LLNEXTGEN),)
 else
 .PHONY: verify-grammar
 
-doc/rust.g: rust.md $(S)src/etc/extract_grammar.py
+doc/rust.g: $(D)/rust.md $(S)src/etc/extract_grammar.py
 	@$(call E, extract_grammar: $@)
 	$(Q)$(CFG_PYTHON) $(S)src/etc/extract_grammar.py $< >$@