nixos/wireguard: Fix mismatched XML tag

Build error introduced in fe7053f75a912197f312d890740dd3bdde0ed994:

  parser error : Opening and ending tag mismatch: commmand line 6139 and command
  escription><para>Base64 preshared key generated by <commmand>wg genpsk</command>
                                                                                 ^
Writing "command" with only two "m" fixes building the NixOS manual.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/modules/services/networking/wg-quick.nix

@@ -147,7 +147,7 @@ let
         example = "rVXs/Ni9tu3oDBLS4hOyAUAa1qTWVA3loR8eL20os3I=";
         type = with types; nullOr str;
         description = ''
-          Base64 preshared key generated by <commmand>wg genpsk</command>.
+          Base64 preshared key generated by <command>wg genpsk</command>.
           Optional, and may be omitted. This option adds an additional layer of
           symmetric-key cryptography to be mixed into the already existing
           public-key cryptography, for post-quantum resistance.