Fix inconsistencies in indexes documentation

[ByteHamster]

Description

Fix inconsistencies in indexes documentation. The table creation and insertion used completely different column names. Creation used customer_id, while insertion used playerid. Also, there was a typo in uppder_customer_id (extra d) and a space too much in upper _customer_id.

I went for playerid and upper_playerid because the other examples on the page use this name as well. Note that the other examples on the page are a bit inconsistent as well, but at least they are not step-by-step instructions like in this example on virtual columns.

Note:
Apart from the inconsistency, there is another possible problem with that documentation section. The heading is "using virtual columns", but the table seems to only have normal columns, no virtual ones. The ingestion seems to explicitly convert and store the value when selecting UPPER(playerid) AS upper_playerid. In contrast, at least in other databases, the column should not be filled when inserting, but filled automatically by defining the column as something like upper_playerid TEXT GENERATED ALWAYS AS (UPPER(playerid)) VIRTUAL during table creation.
This problem is independent of the naming inconsistencies and needs to be fixed by someone who knows the specific syntax you are using for creating virtual columns.

When should this PR be released to the public?

immediate release

Documentation Checklist

• I've previewed my documentation locally running make start-local (or using this tutorial)
• I've validated that indexing works and that I'm able to navigate to the documentation page from the table of contents

If this PR touches a function implementation (aggregate, scalar, or table-valued):

• I've made sure my documentation is aligned with these guidelines on function documentation
  • I'm getting a 404 on that link
• I've validated that the parent of my docs page is set correctly and the function shows up in the right category of the table of contents
• I've made sure that the function was added to the function glossary

[nanimal-fb]

This page needs a general rewrite, which will begin shortly.

[nanimal-fb]

Thank you for paying attention and submitting your corrections. I have incorporated them (in a separate pull request) into our documentation where they are now public: https://docs.firebolt.io/Guides/working-with-indexes/using-primary-indexes.html.