billing: Fix incorrect variable names in test_compute_plan_parameters.
billing: Fix incorrect variable names in test_compute_plan_parameters.

Seems to have introduced in e883567146204630e0273f5080c711ce60bcce42

eeshangarg/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,kou/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip