Fix typo in docstring

[LilithHafner]

The primary change here is:

...snoop out all methods that we defined by us, and not by another package...

to

...snoop out all methods that were defined by us, and not by another package...

My editor also automatically stripped out trailing whitespace on file save, I hope that's okay. I can put back the trailing whitespace if necessary.

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/LilithHafner/Pluto.jl", rev="lh/doc-typo")
julia> using Pluto

[pankgeorg]

Thanks for contributing! I'm not sure what our policy on whitespace is (it kind of messes up the git history); but if this PR only had the one changed line I'll approve it right away!

[pankgeorg]

Thanks! You can make a new white space PR if you want and we can discuss that there! Or maybe even a formatter one. (It would be super cool if the commit message for each line referenced the previous commit of each line, but I don't know if there is any tool that actually does that)