format_doc_comments -> format_code_in_doc_comment

This unstable feature has been renamed (See
rust-lang/rustfmt#3348).

Author: elrnv

rustfmt.toml

@@ -3,7 +3,7 @@ merge_imports = true
 reorder_impl_items = true
 use_field_init_shorthand = true
 use_try_shorthand = true
-format_doc_comments = true
+format_code_in_doc_comment = true
 wrap_comments = true
 edition = "2018"
 version = "Two"