Clarify what creating a gist named keybase.md means

Since a gist can potentially contain several files, it is, at least for people who don't regularly make gists, not immediately clear that `keybase.md` should be entered as the name of the file, rather than the gist description. I guessed correctly while creating my github proof, but did have to guess.

Review the GitHub proof instructions that tell users to create a gist named keybase.md. Clarify that keybase.md is the filename within the gist, not the gist description, and confirm the revised wording is understandable during gist creation.

Written by the indexing model from the issue text.