Fix footnote rendering with mdbook-epub

mdbook-epub 0.4.37 will collapse the second footnote into the first one.
This does not happen with HTML output from plain mdbook.

(cherry picked from commit bc9bd13)

pills/20-basic-dependencies-and-hooks.md

@@ -273,4 +273,5 @@ and if one dependency has that setup hook then all of them will be so `echo`ed.
 ...I'm not sure! We could talk about the additional dependency types and hooks which cross compilation necessitates, building on our knowledge here to cover stdenv as it works today. We could talk about how nixpkgs is bootstrapped. Or we could talk about how `localSystem` and `crossSystem` are elaborated into the `buildPlatform`, `hostPlatform`, and `targetPlatform` each bootstrapping stage receives. Let us know which most interests you!
 
 [^1]: We can now be precise and consider what `addToEnv` does alone the minimal treatment of a dependency: i.e. a package that is _just_ a dependency would _only_ have `addToEnv` applied to it.
+
 [^2]: It was called [GCC Wrapper](https://github.com/NixOS/nixpkgs/tree/6675f0a52c0962042a1000c7f20e887d0d26ae25/pkgs/build-support/gcc-wrapper) in the version of nixpkgs suggested for following along in this pill; Darwin and Clang support hadn't yet motivated the rename.