nested_except_trace: remove indendation from example

... to facilitate copy and paste in a REPL.

my2to3/fixes/fix_nested_except_trace.py

@@ -9,25 +9,25 @@ class FixNestedExceptTrace(BaseStaticTraceFix):
 
   Try the following two snippets yourself in Python 2 and Python 3...
 
+try:
+  raise Exception("foo")
+except Exception as e:
   try:
-    raise Exception("foo")
+    raise Exception("bar")
   except Exception as e:
-    try:
-      raise Exception("bar")
-    except Exception as e:
-      print(e)
     print(e)
+  print(e)
 
   ---
 
+try:
+  raise Exception("foo")
+except Exception as e:
   try:
-    raise Exception("foo")
+    raise Exception("bar")
   except Exception as e:
-    try:
-      raise Exception("bar")
-    except Exception as e:
-      print(e)
-    raise
+    print(e)
+  raise
   """
   basename = "nested_except"
   traced_information = "lineno_parent", "lineno_child"