docs/release-calendar.rsv: Remove spaces

The generated entries don't have spaces, and the csv parser doesn't
like that some rows do and others don't have spaces.

Part-of: <!8356>

docs/release-calendar.csv

-20.3, 2021-01-13, 20.3.3, Dylan Baker,
-    , 2021-01-27, 20.3.4, Dylan Baker,
+20.3,2021-01-13,20.3.3,Dylan Baker,
+,2021-01-27,20.3.4,Dylan Baker,
 21.0,2021-01-13,21.0.0-rc1,Dylan Baker
 ,2021-01-20,21.0.0-rc2,Dylan Baker
 ,2021-01-27,21.0.0-rc3,Dylan Baker