fixes typo

django_auth_lti/verification.py

@@ -12,7 +12,7 @@ def is_allowed(request, allowed_roles, raise_exception):
     if not isinstance(lti_launch, dict):
         # If this is raised, then likely the project doesn't have
         # the correct settings or is being run outside of an lti context
-        raise ImproperlyConfigured("No LTI_LAUNCH vale found in session")
+        raise ImproperlyConfigured("No LTI_LAUNCH found in session")
     user_roles = lti_launch.get('roles', [])
     is_user_allowed =  set(allowed) & set(user_roles)