nixos/oauth2-proxy: fix display-htpasswd-form flag name (#347287)

nixos/modules/services/security/oauth2-proxy.nix

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   } // lib.optionalAttrs (cfg.passBasicAuth) {
     basic-auth-password = cfg.basicAuthPassword;
   } // lib.optionalAttrs (cfg.htpasswd.file != null) {
-    display-htpasswd-file = cfg.htpasswd.displayForm;
+    display-htpasswd-form = cfg.htpasswd.displayForm;
   } // lib.optionalAttrs tls.enable {
     tls-cert-file = tls.certificate;
     tls-key-file = tls.key;