contributing: fix awkward language [ci skip]
This is more of an OCD fix than anything, but technically the canonical
source tree is not located 'in' the master branch; it *is* the master
branch.
@@ -40,7 +40,7 @@ $ urbit my-fake-zod
### Contributing
-The canonical source tree is located in the `master` branch at
+The canonical source tree is the `master` branch of
[https://github.com/urbit/urbit][repo]. You should typically branch off of
`master` when commencing new work; similarly, when we pull in your
contribution, we'll do so by merging it to `master`.
