[AIRFLOW-XXX] Replaces incorrect env var name in INSTALL (#3788)

INSTALL mis-named AIRFLOW_GPL_UNIDECODE environment variable.

Author: PointyShinyBurning

INSTALL

@@ -20,7 +20,7 @@ source my_env/bin/activate
 # To make sure not to install the GPL dependency:
 #   export SLUGIFY_USES_TEXT_UNIDECODE=yes
 # In case you do not mind:
-#   export GPL_UNIDECODE=yes
+#   export AIRFLOW_GPL_UNIDECODE=yes
 
 # [required] building and installing
 # by pip (preferred)