Update oo-design.md

Fix assertion failure message. This also needs to be changed in the source of the d2l.Module class. I'll submit a separate pull request for that.

Author: abhiqaz18

chapter_linear-regression/oo-design.md

@@ -194,7 +194,7 @@ class Module(d2l.nn_Module, d2l.HyperParameters):  #@save
         raise NotImplementedError
 
     def forward(self, X):
-        assert hasattr(self, 'net'), 'Neural network is defined'
+        assert hasattr(self, 'net'), 'Neural network is not defined'
         return self.net(X)
 
     def plot(self, key, value, train):