Fixing path of environment build in getting-started notebook.

notebooks/getting-started.ipynb

    "source": [
     "### 1. Set environment parameters\n",
     "\n",
-    "Be sure to set `env_name` to the name of the Unity environment file you want to launch. Ensure that the environment build is in the ML-Agents SDK directory."
+    "Be sure to set `env_name` to the name of the Unity environment file you want to launch. Ensure that the environment build is in `../envs`."
    ]
   },
   {