Big docs bug fix

"tau only required if sd not specified."

Previously was "tau only required if tau not specified."

Author: ericmjl

pymc3/distributions/continuous.py

@@ -270,7 +270,7 @@ class Normal(Continuous):
     sd : float
         Standard deviation (sd > 0) (only required if tau is not specified).
     tau : float
-        Precision (tau > 0) (only required if tau is not specified).
+        Precision (tau > 0) (only required if sd is not specified).
         
     Examples
     --------