rustfmt: do not use unstable features

Sadly, `rustfmt` does not respect `RUSTC_BOOTSTRAP=1` yet,
so we will have to avoid them for the moment.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

.rustfmt.toml

@@ -1,11 +1,11 @@
 edition = "2018"
-format_code_in_doc_comments = true
 newline_style = "Unix"
 
 # Unstable options that help catching some mistakes in formatting and that we may want to enable
 # when they become stable.
 #
 # They are kept here since they are useful to run from time to time.
+#format_code_in_doc_comments = true
 #reorder_impl_items = true
 #comment_width = 100
 #wrap_comments = true