Revert detachedExit -> exit change

Author: kleisauke

src/library_pthread.js

@@ -304,7 +304,7 @@ var LibraryPThread = {
         worker.on('error', function(e) {
           worker.onerror(e);
         });
-        worker.on('exit', function() {
+        worker.on('detachedExit', function() {
           // TODO: update the worker queue?
           // See: https://github.com/emscripten-core/emscripten/issues/9763
         });