[gnome-software] RELEASE.md: Add -Dsoup2=true to the "Build the release tarball" meson command

[Milan Crha <mcrha src gnome org>]

commit 8887e4eb96f4a7d8b0144196f7b24e2bb410c10e
Author: Milan Crha <mcrha redhat com>
Date:   Fri Jan 7 09:47:31 2022 +0100

    RELEASE.md: Add -Dsoup2=true to the "Build the release tarball" meson command
    
    This is until fully switched to libsoup3.

 RELEASE.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/RELEASE.md b/RELEASE.md
index f310962a8..d1a80e390 100644
--- a/RELEASE.md
+++ b/RELEASE.md
@@ -57,7 +57,7 @@ Build the release tarball:
 # Only execute git clean if you don't have anything not tracked by git that you
 # want to keep
 git clean -dfx
-meson --prefix $PWD/install -Dbuildtype=release build/
+meson --prefix $PWD/install -Dbuildtype=release -Dsoup2=true build/
 ninja -C build/ dist
 ```