Fix documentation typo

The documentation says "create 9 random times" but the code shows 6 random times. Updated documentation to reflect the code.

doc/notebooks/sgp4_partial_derivatives.ipynb

@@ -593,7 +593,7 @@
     "    data.append(lines[i+1])\n",
     "    data.append(lines[i+2])\n",
     "    tles.append(dsgp4.tle.TLE(data))\n",
-    "#we also create 9 random times, tracking the gradients:\n",
+    "#we also create 6 random times, tracking the gradients:\n",
     "tsinces=torch.rand((6,))"
    ]
   },