Use same cancelled spelling in doc and code.

Right thing might be to update the spelling in the code to
follow American instead of English spelling, that is
using only canceled. But they should at least be aligned.

Author: mattiasgronlund

docs/dev/guide.md

@@ -48,10 +48,10 @@ changes "uniquely", but we might also want to fork an `Analysis` and send it to
 another thread for background processing. That is, there is only a single
 `AnalysisHost`, but there may be several (equivalent) `Analysis`.
 
-Note that all of the `Analysis` API return `Cancelable<T>`. This is required to
+Note that all of the `Analysis` API return `Cancellable<T>`. This is required to
 be responsive in an IDE setting. Sometimes a long-running query is being computed
 and the user types something in the editor and asks for completion. In this
-case, we cancel the long-running computation (so it returns `Err(Canceled)`),
+case, we cancel the long-running computation (so it returns `Err(Cancelled)`),
 apply the change and execute request for completion. We never use stale data to
 answer requests. Under the cover, `AnalysisHost` "remembers" all outstanding
 `Analysis` instances. The `AnalysisHost::apply_change` method cancels all