docs: initrd support is not released yet

When firecracker-microvm#1246 was merged,
the changelog entry was added under 0.19.0. This seems like a mistake as 0.19.0
was already released.

Signed-off-by: Angus McInnes <angus@amcinnes.info>

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## [Unreleased]
 
+### Added
+- Support for booting with an initial RAM disk image. This image can be
+  specified through the new `initrd_path` field of the `/boot-source` API
+  request.
+
 ### Fixed
 
 - Fixed #1469 - Broken GitHub location for Firecracker release binary.
@@ -56,9 +61,6 @@
   all parameters specified after `--` are forwarded verbatim to Firecracker.
 - Added `KVM_PTP` support to the recommended guest kernel config.
 - Added entry in FAQ.md for Firecracker Guest timekeeping.
-- Support for booting with an initial RAM disk image. This image can be
-  specified through the new `initrd_path` field of the `/boot-source` API
-  request.
 
 ### Changed