SQL tutorial ex3 fixes

[mcbex]

I noticed exercise 3 in the intro to SQL tutorial was broken. The tutorial and exercise notebooks still reference the BigQuery public hacker news dataset comments table. This dataset has been updated and now only contains one table: full.

The first query in the exercise doesn't work as nicely using the full table because one of the columns used is called by and needs to be quoted. However this PR attempts to fix the issue by referencing the correct table and adding a note in the tutorial about reserved words.

Related discussion post on Kaggle:
https://www.kaggle.com/learn/intro-to-sql/discussion/417044#2316394

Copy of the tutorial notebook with proposed changes applied:
https://www.kaggle.com/mmcbex/sql-tutorial-3-updated

Fixes:
#452

A couple follow up issues related to this one:

• In tutorial 1, the hacker news dataset is also explored and the missing tables are mentioned. Unfortunately there is a nice diagram that is now outdated.
• Exercises 1 and 2 in the Intro to SQL course add the hacker news dataset but do not use them. I believe those could be removed by deleting the references in this file but maybe out of scope for this PR.
• An updated version of the dataset could be published on Kaggle. I will report the dataset as outdated and include a like to this PR for context (my report failed with an internal error 🤷‍♀️).

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[pculliton]

Thank you, Ming! We really appreciate this PR and your contribution.