Fixup comment on unusual cfg_attr

Co-authored-by: waffle <[email protected]>

Author: workingjubilee

library/std/src/lib.rs

@@ -273,7 +273,7 @@
 // Language features:
 // tidy-alphabetical-start
 
-// because this actually got to beta without being noticed
+// stabilization was reverted after it hit beta
 #![cfg_attr(not(bootstrap), feature(extended_varargs_abi_support))]
 #![feature(alloc_error_handler)]
 #![feature(allocator_internals)]