Ensure we spell Actor with capital A

[honzajavorek]

This task is also an opportunity for me to learn more about configuring vale and writing rules for it.

• This is the rule file https://github.com/apify/apify-docs/blob/master/.github/styles/Apify/Capitalization.yml The RE doesn't work correctly now.
• This is the config file https://github.com/apify/apify-docs/blob/master/vale.ini The rule seems to be ON.

The consensus is that Actor should be spelled with capital A. I think I could then go and change it everywhere in the Academy.