Rename the resource to b64url_st

Vestiges when this was in an app named couch_seqs.
diff --git a/c_src/b64url.c b/c_src/b64url.c
index 604f21a..478468b 100644
--- a/c_src/b64url.c
+++ b/c_src/b64url.c
@@ -253,7 +253,7 @@
     }
 
     res = enif_open_resource_type(
-            env, NULL, "couch_seq", b64url_st_free, flags, NULL);
+            env, NULL, "b64url_st", b64url_st_free, flags, NULL);
     if(res == NULL) {
         return 1;
     }