DOC: Remove Sphinx dependency from RTD example config

... because nbsphinx already depends on it.

Author: mgeier

doc/usage.ipynb

@@ -165,7 +165,6 @@
     "\n",
     "1. Create a file named `requirements.txt` (or whatever name you wish) in your repository containing the required pip packages:\n",
     "\n",
-    "        sphinx>=1.4\n",
     "        ipykernel\n",
     "        nbsphinx\n",
     "        \n",
@@ -197,7 +196,6 @@
     "          - conda-forge\n",
     "        dependencies:\n",
     "          - python==3.5\n",
-    "          - sphinx>=1.4\n",
     "          - pandoc\n",
     "          - nbconvert\n",
     "          - ipykernel\n",