Update EMSCRIPTEN.md

Blockstream/blind_pin_server#16 (comment)

EMSCRIPTEN.md

@@ -2,5 +2,5 @@
 ```./emscripten.sh```
 
 # To test run the following
-```python -m http.server 9999``` in the repo root directory and open ```http://127.0.0.1:9990``` in your browser
+```python -m http.server 9999``` in the repo root directory and open ```http://127.0.0.1:9999``` in your browser