Fix typo in ReceivePack.java

The comment indicates that a well-behaved client should not have
sent an update for a ref that already exists, but this in a block
that corresponds to a create command.

org.eclipse.jgit/src/org/eclipse/jgit/transport/ReceivePack.java

@@ -669,8 +669,8 @@ private void validateCommands() {
 				}
 
 				if (ref != null) {
-					// A well behaved client shouldn't have sent us an
-					// update command for a ref we advertised to it.
+					// A well behaved client shouldn't have sent us a
+					// create command for a ref we advertised to it.
 					//
 					cmd.setResult(Result.REJECTED_OTHER_REASON, "ref exists");
 					continue;