nested_except_trace: remove indendation from example

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

my2to3/fixes/fix_nested_except_trace.py

@@ -9,9 +9,9 @@ class FixNestedExceptTrace(BaseStaticTraceFix):
 
   Try the following two snippets yourself in Python 2 and Python 3...
 
-  try:
+try:
   raise Exception("foo")
-  except Exception as e:
+except Exception as e:
   try:
     raise Exception("bar")
   except Exception as e:
@@ -20,9 +20,9 @@ class FixNestedExceptTrace(BaseStaticTraceFix):
 
   ---
 
-  try:
+try:
   raise Exception("foo")
-  except Exception as e:
+except Exception as e:
   try:
     raise Exception("bar")
   except Exception as e: