typos in geometric descriptions in the docs

[just-walk]

Unless I'm misunderstanding, it appears that "h" and "f" have been interchanged in the geometric description of the pushforward (Line 73 of propagators.md, specifically). If so, then that document could probably use someone knowledgeable to edit it for clarity. Even with some familiarity with differential geometry, the math is confusing. E.g. which functions are defined on what spaces.