Fix compiler warning (const_item_mutation)

[webmaster128]

This comes up when compiling with a newer compiler (cosmwasm/workspace-optimizer:0.10.3 -> cosmwasm/workspace-optimizer:0.10.4)

   Compiling cw3-fixed-multisig v0.3.0 (/code/contracts/cw3-fixed-multisig)
warning: taking a mutable reference to a `const` item
   --> contracts/cw3-fixed-multisig/src/contract.rs:169:5
    |
169 |     BALLOTS.update(
    |     ^^^^^^^
    |
    = note: `#[warn(const_item_mutation)]` on by default
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
note: `const` item defined here
   --> contracts/cw3-fixed-multisig/src/state.rs:63:1
    |
63  | pub const BALLOTS: Map<(U64Key, &[u8]), Ballot> = Map::new(b"votes");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: 1 warning emitted