add TruncatedNormal to initialize.py (#583)

Update initialize.py

Update initialize.py for proper TruncatedNormal importing

brainpy/initialize.py

@@ -22,6 +22,7 @@
 from brainpy._src.initialize.random_inits import (
   Normal as Normal,
   Uniform as Uniform,
+  TruncatedNormal as TruncatedNormal,
   VarianceScaling as VarianceScaling,
   KaimingUniform as KaimingUniform,
   KaimingNormal as KaimingNormal,