fix typo

Author: mr-c

cwltool/draft2tool.py

@@ -183,7 +183,7 @@ def makeJobRunner(self, use_container=True):  # type: (Optional[bool]) -> JobBas
                 if t["class"] == "DockerRequirement":
                     raise UnsupportedRequirement(
                         "--no-container, but this CommandLineTool has "
-                        "DockerRequiremen under 'requirements'.")
+                        "DockerRequirement under 'requirements'.")
             return CommandLineJob()
 
     def makePathMapper(self, reffiles, stagedir, **kwargs):