upgrade to docify v0.2.4 (#1653)

Upgrades to docify v0.2.4 which adds support for exporting trait and
impl items in addition to regular items. This fixes @liamaharon's
https://github.com/sam0x17/docify/issues/9 issue.

See the release notes for more information:
https://github.com/sam0x17/docify/releases/tag/v0.2.4

substrate/frame/fast-unstake/Cargo.toml

@@ -27,7 +27,7 @@ frame-election-provider-support = { path = "../election-provider-support", defau
 
 frame-benchmarking = { path = "../benchmarking", default-features = false, optional = true}
 
-docify = "0.2.3"
+docify = "0.2.4"
 
 [dev-dependencies]
 pallet-staking-reward-curve = { path = "../staking/reward-curve" }