Fix error formatting bug. (#448)

changelogs/fragments/448-docker_stack-error.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "docker_stack - fix broken string formatting when reporting error in case ``compose`` was containing invalid values (https://github.com/ansible-collections/community.docker/pull/448)."

plugins/modules/docker_stack.py

@@ -246,8 +246,7 @@ def main():
             elif isinstance(compose_def, string_types):
                 compose_files.append(compose_def)
             else:
-                module.fail_json(msg="compose element '%s' must be a " +
-                                 "string or a dictionary" % compose_def)
+                module.fail_json(msg="compose element '%s' must be a string or a dictionary" % compose_def)
 
         before_stack_services = docker_stack_inspect(module, name)