Update go.mod

[mholt]

Fix this error:

go: github.com/nguyer/promptui@v0.8.1-0.20210517132806-70ccd4709797: parsing go.mod:
	module declares its path as: github.com/manifoldco/promptui
	        but was required as: github.com/nguyer/promptui

[mholt]

Gentle reminder about this :) If you could please review and merge, thank you!