message: Fix typo in C++ doc
diff: @@ -526,7 +526,7 @@ Creating a FlatBuffer is not thread safe. All state related to building
  a FlatBuffer is contained in a FlatBufferBuilder instance, and no memory
  outside of it is touched. To make this thread safe, either do not
  share instances of FlatBufferBuilder between threads (recommended), or
  -manually wrap it in synchronisation primites. There's no automatic way to
  +manually wrap it in synchronisation primitives. There's no automatic way to
  accomplish this, by design, as we feel multithreaded construction
  of a single buffer will be rare, and synchronisation overhead would be costly.
  