internal error: fix a few typos

[retokromer]

Summary of changes

Fixes three typos in the internal_error messages.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have confirmed my fix is effective or that my feature works

[piste-jp]

LGTM!