[mingw] Update instructions

https://github.com/harfbuzz/harfbuzz/issues/1728

RELEASING.md

diff --git a/RELEASING.md b/RELEASING.md
index bcc6390..360aea7 100644
--- a/RELEASING.md
+++ b/RELEASING.md
@@ -48,7 +48,7 @@
 
    a. Build Win32 binaries.  See [README.mingw.md](README.mingw.md).
 
-   b. TODO.
+   b. Run "make dist-win" to build Win32 bundle.
 
 11. Copy all artefacts to users.freedesktop.org and move them into
     `/srv/www.freedesktop.org/www/software/harfbuzz/release` There should be four