Command/Multicast: use proper hyphen

Command/Multicast: use proper hyphen

GHC was complaining about it possibly being a homoglyph:

Command/Multicast.hs:111:36: error:
     warning: treating Unicode character <U+2212> as identifier character rather than as '-' symbol [-Wunicode-homoglyph]
            -- using a nice prime, namely 2521−1  but the sheer size of this
                                              ^
    |
111 |         -- using a nice prime, namely 2521−1  but the sheer size of this
    |                                    ^
1 warning generated.