Encryption - fix typos and improve clarity

[ademidoff]

This pull request makes minor improvements to the documentation for PMM's data encryption configuration. The changes clarify instructions and correct some wording to improve readability.

• Documentation clarity:
  • Corrected the wording to refer to a "custom key" instead of "custom keys" and clarified that the environment variable should be set before upgrading to PMM 3 or starting a new PMM 3.x instance.
  • Improved phrasing to state that PMM uses "the custom key" for encryption and decryption, rather than "custom keys".