Fix start date in 3228

Author: xzmeng

text/3228-process-process_group.md

@@ -1,5 +1,5 @@
 - Feature Name: `process_set_process_group`
-- Start Date: 02-02-2022
+- Start Date: 2022-02-02
 - RFC PR: [rust-lang/rfcs#3228](https://github.com/rust-lang/rfcs/pull/3228)
 - Rust Issue: [rust-lang/rust#93857](https://github.com/rust-lang/rust/issues/93857)