rust/docs: fix critical typo for missing_docs lint

Fix typo from fe66d06a45a4714141eba992fe87ec3dd5fa1c22.
The exclamation point is what lets an attribute apply to
an entire crate, without the ! it's practically a placebo.

Fix on commits af182d4ab51d6a1a70559bbdcd4ab842aa855684 and
b6059297d7cb76f0e00e2098e38d6677d3033340, and note there are
still missing docs in both crypto and protover, for now.

https://doc.rust-lang.org/reference/attributes.html

doc/HACKING/CodingStandardsRust.md

@@ -104,7 +104,7 @@ repo.
  Documentation
 ---------------
 
-You MUST include `#[deny(missing_docs)]` in your crate.
+You MUST include `#![deny(missing_docs)]` in your crate.
 
 For function/method comments, you SHOULD include a one-sentence, "first person"
 description of function behaviour (see requirements for documentation as

src/rust/crypto/lib.rs

@@ -24,7 +24,8 @@
 //! assert!(result == [b'X'; DIGEST256_LEN]);
 //! ```
 
-#[deny(missing_docs)]
+// XXX: add missing docs
+//#![deny(missing_docs)]
 
 // External crates from cargo or TOR_RUST_DEPENDENCIES.
 extern crate digest;

src/rust/protover/lib.rs

@@ -22,7 +22,8 @@
 //! protocols to develop independently, without having to claim compatibility
 //! with specific versions of Tor.
 
-#[deny(missing_docs)]
+// XXX: add missing docs
+//#![deny(missing_docs)]
 
 extern crate libc;
 extern crate smartlist;