Rename mrms importer

it'll make sense once #154 is merged

Author: dnerini

examples/my_first_nowcast.ipynb

@@ -238,7 +238,7 @@
     "\n",
     "- `fn_ext`: 'grib2' -- The file extension\n",
     "- `fn_pattern`: 'PrecipRate_00.00_%Y%m%d-%H%M%S' -- The file naming convention of the MRMS data.\n",
-    "- `importer`: 'mrms' -- The used importer, which is for the MRMS data.\n",
+    "- `importer`: 'mrms_grib' -- The name of the importer for the MRMS data.\n",
     "- `importer_kwargs`: {} -- Extra options provided to the importer. None in this example.\n",
     "- `path_fmt`: '%Y/%m/%d' -- The folder structure in which the files are stored. Here, year/month/day/filename.\n",
     "- `root_path`: '/content/pysteps_data/mrms' -- The root path of the MRMS-data.\n",