btrfs-progs: ci: fix image updater script

There's a typo in the way how parameter is passed and is a syntax error
for docker.

Signed-off-by: David Sterba <dsterba@suse.com>

ci/images/images-base-update

@@ -4,6 +4,6 @@
 for dir in ci-*; do
 	echo "Update $dir"
 	cd "$dir"
-	docker pull `awk '/^FROM/ {print $$2}' < Dockerfile`
+	docker pull `awk '/^FROM/ {print $2}' < Dockerfile`
 	cd ..
 done