don't use unstable feature on rustfmt yet

rustfmt.toml

@@ -1,10 +1,12 @@
-comment_width = 100
-doc_comment_code_block_width = 100
-format_code_in_doc_comments = true
-group_imports = "StdExternalCrate"
-imports_granularity = "Crate"
+# Uncomment these when they become stabilized by Rust
+#comment_width = 100
+#doc_comment_code_block_width = 100
+#format_code_in_doc_comments = true
+#group_imports = "StdExternalCrate"
+#imports_granularity = "Crate"
+#wrap_comments = true
+
 max_width = 100
 use_small_heuristics = "Max"
-wrap_comments = true
 # Workaround for https://github.com/rust-lang/rust.vim/issues/464
 edition = "2021"