DOC: to_datetime outdated example
With the new default of `errors='raise'`, the following section in the docs is not fully correctly phrased anymore: http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#invalid-data. 

So I just removed that example of `to_datetime([1, '1'])` (as it now raises, which is less unexpected, and the new example already shows the three options).
