Update bbb-install-2.6.sh

Apply fix to generate secret for GreenLight

Author: ffdixon

bbb-install-2.6.sh

@@ -896,7 +896,7 @@ install_greenlight_v3(){
 
   local PGDBNAME=greenlight-v3-production
   local SECRET_KEY_BASE
-  SECRET_KEY_BASE=$(docker run --rm --entrypoint bundle $GL_IMG_REPO exec rake secret)
+  SECRET_KEY_BASE=$(docker run --rm --entrypoint bundle $GL_IMG_REPO exec rails secret)
 
   if [ -z "$SECRET_KEY_BASE" ]; then
     err "failed to generate greenlight-v3 secret key base - is docker running?"