Add missing commas to rustc_ast_pretty::pp docs

Author: camelid

compiler/rustc_ast_pretty/src/pp.rs

@@ -75,15 +75,15 @@
 //! breaking inconsistently to become
 //!
 //! ```
-//! foo(hello, there
+//! foo(hello, there,
 //!     good, friends);
 //! ```
 //!
 //! whereas a consistent breaking would yield:
 //!
 //! ```
 //! foo(hello,
-//!     there
+//!     there,
 //!     good,
 //!     friends);
 //! ```