update nightly book

Author: mark-i-m

src/doc/unstable-book/src/language-features/macro-at-most-once-rep.md

@@ -1,13 +1,15 @@
 # `macro_at_most_once_rep`
 
-The tracking issue for this feature is: TODO(mark-i-m)
+NOTE: This feature is only available in the 2018 Edition.
+
+The tracking issue for this feature is: #48075
 
 With this feature gate enabled, one can use `?` as a Kleene operator meaning "0
 or 1 repetitions" in a macro definition. Previously only `+` and `*` were allowed.
 
 For example:
 
-```rust
+```rust,ignore
 #![feature(macro_at_most_once_rep)]
 
 macro_rules! foo {