Fix DeprecationWarning in mitogen.utils.run_with_router()

```
14:20:32.186270 W py.warnings:
/home/alex/src/mitogen/mitogen/utils.py:152: DeprecationWarning: invalid
escape sequence \*
  """
```

docs/changelog.rst

@@ -438,6 +438,9 @@ Core Library
 * `#523 <https://github.com/dw/mitogen/issues/523>` : the test suite didn't
   generate a code coverage report if any test failed.
 
+* `#524 <https://github.com/dw/mitogen/issues/524>` : Python 3.6+ emitted a
+  :class:`DeprecationWarning` for :func:`mitogen.utils.run_with_router`.
+
 
 Thanks!
 ~~~~~~~

mitogen/utils.py

@@ -143,7 +143,7 @@ def log_to_file(path=None, io=False, level='INFO'):
 
 def run_with_router(func, *args, **kwargs):
     """
-    Arrange for `func(router, \*args, \**kwargs)` to run with a temporary
+    Arrange for `func(router, *args, **kwargs)` to run with a temporary
     :class:`mitogen.master.Router`, ensuring the Router and Broker are
     correctly shut down during normal or exceptional return.