fix minor grammar mistake

Author: simon-lammes

src/send-and-sync.md

@@ -65,7 +65,7 @@ impl !Sync for SpecialThreadToken {}
 
 Note that *in and of itself* it is impossible to incorrectly derive Send and
 Sync. Only types that are ascribed special meaning by other unsafe code can
-possible cause trouble by being incorrectly Send or Sync.
+possibly cause trouble by being incorrectly Send or Sync.
 
 Most uses of raw pointers should be encapsulated behind a sufficient abstraction
 that Send and Sync can be derived. For instance all of Rust's standard