Fix typo in reserverd words

[gionata]

natual -> natural

[rlaiola]

Ciao, @gionata.

The scope of this PR changed quite a bit and it would be interesting to document it. An overview shows that it now integrates some of the open PRs, that are:

• Relax API support (new features + bug fix) #148
• Fix concat expression #178
• Add datasets used in db courses at the Federal University of Espírito Santo (Ufes, Brazil) #194
• Add project all columns (π *) to relational and multiset algebra #203
• Allow using local table/variable name to reference columns in relational algebra #205
• Add support for TableDum and TableDee (Tutorial D) #212
• Add devcontainer, linters and formatters #213
• Consolidate PRs on Bags/Multiset Algebra #215
• Fix distorted execution tree in complex queries and jpg download #216
• 🐛 fix date check crash #217
• Italian translation #225

As this branch was merged from https://github.com/rlaiola/relax/tree/main/, which I use in my database courses at Ufes (including for offline exams), you might reconsider the following changes:

• Restore deleted Gist groups: https://github.com/dbis-uibk/relax/pull/224/files#diff-1c5609e0dc3eb0ea42c2f7988db2f07bf5980e5d371d5a347dd4317c1a25f6eb
• Set uibk as the default group: https://github.com/dbis-uibk/relax/pull/224/files#diff-e6c43a725da6b7c65c809a4f62cdeff483ff8c30bedf5905b1631960902524d6
• Restore Google Analytics: https://github.com/dbis-uibk/relax/pull/224/files#diff-d1f6784786bdac447110e8f721202e0c4c56cd820d97fbc97c1f50f4d7175836

Hope it helps!