Add dni to example variable in doc

Author: AdamRJensen

pvlib/iotools/era5.py

@@ -82,7 +82,7 @@ def get_era5(latitude, longitude, start, end, variables, api_key,
         Last day of the requested period.  Assumed to be UTC if not localized.
     variables : list of str
         List of variable names to retrieve, for example
-        ``['ghi', 'temp_air']``. Both pvlib and ERA5 names can be used.
+        ``['ghi', 'dni', 'temp_air']``. Both pvlib and ERA5 names can be used.
         See [1]_ for additional options.
     api_key : str
         ECMWF CDS API key.