Reduce prune threshold in track to track fusion example

On certain environments, no tracks are formed as mixture is pruned too
soon.

docs/examples/Track2Track_Fusion_Example.py

@@ -461,7 +461,7 @@
 # The states tend to have low weights when they are first initialized using this method, so we will
 # keep the pruning threshold low.
 ch_reducer = GaussianMixtureReducer(
-    prune_threshold=1E-9,
+    prune_threshold=1E-10,
     pruning=True,
     merge_threshold=200,
     merging=True