Update gpg.md

What does this MR do and why?

Update gpg.md

Add another error message encountered when GPG_TTY is not set. I encountered this error when trying to sign a commit. The fix is the same "GPG failed to sign the data", so I thought it made sense to add the error to the list there instead of adding a new section with the same steps.

gpg: signing failed: Inappropriate ioctl for device

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