fixed ws import

Double-Couple-Waveform-Inversion.ipynb

@@ -12,7 +12,7 @@
     "from pyrocko import gf, moment_tensor as mtm, trace\n",
     "import scipy\n",
     "import matplotlib.pyplot as plt\n",
-    "from pyrocko.gf import LocalEngine, Target, DCSource\n",
+    "from pyrocko.gf import ws, LocalEngine, Target, DCSource\n",
     "from pyrocko.guts import Object, Float, String, Dict, Int\n",
     "from pyrocko import util, pile, model, config, trace, io, pile, catalog\n",
     "import time\n",