verify-grammar keyword crate

When calling

   make verify-grammar

or when llnextgen is not installed:

   python2.7 src/etc/extract_grammar.py <src/doc/rust.md

an error is reported by extract_grammar.py that the

keyword "crate" is not defined.

This patch adds the keyword "crate" to the grammar in rust.md.

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

src/doc/rust.md

@@ -205,6 +205,7 @@ The keywords are the following strings:
 ~~~~ {.notrust .keyword}
 as
 break
+crate
 do
 else enum extern
 false fn for