Fix a typo

Author: tclfs

src/doc/book/src/concurrency.md

@@ -36,7 +36,7 @@ down the channel and to the other thread. Therefore, we'd ensure that `Send` was
 implemented for that type.
 
 In the opposite way, if we were wrapping a library with [FFI][ffi] that isn't
-threadsafe, we wouldn't want to implement `Send`, and so the compiler will help
+thread-safe, we wouldn't want to implement `Send`, and so the compiler will help
 us enforce that it can't leave the current thread.
 
 [ffi]: ffi.html