DOC: Replace astropy channel with conda-forge [ci skip]

Author: pllim

tutorials/notebooks/FITS-cubes/FITS-cubes.ipynb

@@ -35,9 +35,9 @@
    "metadata": {},
    "source": [
     "```\n",
-    "conda install -c astropy astroquery\n",
-    "conda install -c astropy spectral-cube\n",
-    "conda install -c astropy reproject\n",
+    "conda install -c conda-forge astroquery\n",
+    "conda install -c conda-forge spectral-cube\n",
+    "conda install -c conda-forge reproject\n",
     "```"
    ]
   },