Update path to Observatory in HACKING.txt

runtime/bin/vmservice/observatory/HACKING.txt

 Dart Observatory Developer Guide
 ---
 
 Assumptions:
 You are running pub from the latest dev channel release of Dart Editor.
 
 During development you do not need to run dart2js or rebuild the VM (unless
 you are also making changes to the backend). While you're working on your
 feature follow the steps:
 
-1. Open runtime/bin/vmservice/client in the Dart Editor
+1. Open runtime/bin/vmservice/observatory in the Dart Editor
 2. Run pub upgrade
 3. Run dart --observe script.dart
 4. Run index.html in Dartium
 
 At this point you should see the initial Observatory UI and that
 it is communicating with the VM you launched in step 3.
 
 Continue to develop and iterate until you're ready to upload your change
 for review. Upload your change and get an LGTM.
 
 5. Run pub build
 6. Run ./deploy.sh
 
 Note: If you run pub from within the editor you need to make sure that it runs
 in release mode (--mode=release), i.e., output is minified and does not include
 any .dart source files.
 
 At this point you should rebuild your VM and:
 
 7. Launch dart --observe script.dart
 
 In a non-Dart enabled browser navigate to localhost:8181 and ensure
 that your feature works after being compiled to JavaScript.
 
 8. Ensure the vm, standalone, and observatory unit tests still pass:
 python tools/test.py --mode=debug --arch=ia32 --compiler=none --runtime=vm vm/cc standalone/vmservice vmservice
 
 9. Commit your change