Add link to pandas docs on using apply

The data types and formats chapter introduces the use of the apply function in pandas, but does not link to the pandas docs on it. As there are many options to it and things that might be non-obvious to new users (e.g. how it handles arguments to a function) we should add a link to the docs in the rounding exercise (https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.apply.html).