docs: Tweak phrasing about OIDC supporting only a single provider.
docs: Tweak phrasing about OIDC supporting only a single provider.

Strictly speaking, this sentence is talking about the IdP configuration,
while the backend is just GenericOpenIdConnectBackend, so the new
phrasing is more correct.

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