[Rd] Documentation tweak for ?traceback

[brodie gaslam]

The addition of `.traceback` in r70207 adds one more function to the call stack when invoking `traceback()`.  This changes the output of one of the examples to include the error handler call:


> options(error = function() traceback(2))
> foo(2)
[1] 1
Error in bar(2) : object 'a.variable.which.does.not.exist' not found
3: (function () 
   traceback(2))() at #1
2: bar(2) at #1
1: foo(2)

The attached trivial patch cleans up the example so that the above looks like it would have under r<70207.
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