Proto_manager: add the Protocol.directory case after the existing ones.

Proto_manager used to add the corresponding line before the line where source is defined, but it broke the script in the case where the whole function definition stand on one line, and the added case was merged with the number functions cases.

Having the function on one line is probably a recent change due to using the ALPHA NUM NUM NUM pattern for variant instead of short hash.

The line concerned is in tezt/lib_tezos/protocol.ml

When trying to ./scripts/proto_manager.sh --stabilise --from alpha --to beta, you ended up with

| Beta -> "proto_beta"
let directory = function R022 -> "proto_022_PsRiotum" | Alpha -> "proto_alpha"

Now, you'll have

let directory = function R022 -> "proto_022_PsRiotum" | Alpha -> "proto_alpha"
| Beta -> "proto_beta"