minor changes to 08-data-frames

[paulmaxus]

Commit 1 adds print statements on 2 lines of code:
To be consistent with the rest of the episode's code, the dataframes in question should be printed explicitly.
Leaving out the print statement suggests that this would also work outside of the jupyter environment.

Commit 2 changes the explanation of the .loc method:

1. dictionaries were not explained yet in this lesson; I deleted that part
2. location can be specified by column names as well; I've added that information

[alee]

Thanks for the PR! I think these are good incremental improvements though there are further refinements expected to this section from #598