MBE parser is out of date

[JerryQ17]

rust-lang/rust#143070 rewrites macro_rules! parser to not use the MBE engine itself.

The following is the relevant section (39.2 Macro expansion):

rustc-dev-guide/src/macro-expansion.md

Lines 557 to 559 in 0549d8c

	When the compiler comes to a `macro_rules` definition, it uses this pattern to
	match the two token trees per the rules of the definition of the macro, _thereby
	utilizing the macro parser itself_. In our example definition, the