app: 🔖 fix Cargo.toml dependency comment

this list has grown to include other external dependencies, not just
crates in the penumbra-zone organization. change this comment to reflect
that.

crates/core/app/Cargo.toml

@@ -32,7 +32,7 @@ penumbra-distributions = { path = "../component/distributions" }
 penumbra-compact-block = { path = "../component/compact-block" }
 penumbra-transaction = { path = "../transaction", features = ["parallel"] }
 
-# Penumbra dependencies
+# External dependencies
 decaf377 = { version = "0.5" }
 decaf377-rdsa = { version = "0.7" }
 jmt = "0.9"