Fix GCD typo

THREADING.md

@@ -25,9 +25,10 @@ Error messages
 The error message is thread-local. The `giterr_last()` call must
 happen on the same thread as the error in order to get the
 message. Often this will be the case regardless, but if you use
-something like the GDC on MacOS (where code is executed on an
-arbitrary thread), the code must make sure to retrieve the error code
-on the thread where the error happened.
+something like the [GCD](http://en.wikipedia.org/wiki/Grand_Central_Dispatch)
+on Mac OS X (where code is executed on an arbitrary thread), the code
+must make sure to retrieve the error code on the thread where the error
+happened.
 
 Threads and cryptographic libraries
 =======================================