Fix ssreflect warning on Coq 8.10

Since Coq 8.10, `move => {}e` means `move => {e}e`.
For the backward-compatible syntax and discussion, see
https://github.com/coq/coq/issues/10550#issuecomment-542397265.

theories/bi/tactics.v

@@ -81,7 +81,7 @@ Module bi_reflection. Section bi_reflection.
   Lemma flatten_cancel e e' ns :
     cancel ns e = Some e' → flatten e ≡ₚ ns ++ flatten e'.
   Proof.
-    rewrite /cancel fmap_Some=> -[{e'}e' [He ->]]; rewrite flatten_prune.
+    rewrite /cancel fmap_Some=> -[{e'}-e' [He ->]]; rewrite flatten_prune.
     revert e' He; induction ns as [|n ns IH]=> e' He; simplify_option_eq; auto.
     rewrite Permutation_middle -flatten_cancel_go //; eauto.
   Qed.