docs: add @std/collections to stabilization schedule (#4774)

README.md

@@ -67,8 +67,11 @@ documentation:
 The Standard Library is now in the process of stabilizing its packages. Below is
 the current schedule:
 
-- [ ] [`@std/bytes`](https://jsr.io/@std/bytes) is being stabilized on July 7,
- 2024 ([#4746](https://github.com/denoland/deno_std/pull/4746))
+- [ ] [`@std/bytes`](https://jsr.io/@std/bytes) is being stabilized on June 7,
+ 2024 ([#4746](https://github.com/denoland/deno_std/pull/4746)).
+- [ ] [`@std/collections`](https://jsr.io/@std/collections) is being stabilized
+ on June 20, 2024
+ ([#4773](https://github.com/denoland/deno_std/pull/4773)).
 
 Each package must go through the following steps to achieve stabilization: