Use type that is mapped to ParseChangelogErrorInternal::Markdown (#4)

The value of the error being mapped here
https://github.com/heroku/keep_a_changelog/blob/c4d62b03052a0fd5f17424fcad4c646abae01d31/src/changelog.rs#L270
isn't a String anymore, which causes compilation errors.

The default behavior of the `markdown` crate was changed in
wooorm/markdown-rs@3dc7f1a.

Alternative solution would be pinning the `markdown` crate to version
[1.0.0-alpha.16](https://github.com/wooorm/markdown-rs/releases/tag/1.0.0-alpha.16).
Seems like it might be a good idea to pin the version in any case though
(considering breaking changes are introduced like this?)

Cargo.toml

@@ -8,7 +8,7 @@ rust-version = "1.74"
 chrono = "0.4"
 indexmap = "2"
 lazy_static = "1"
-markdown = "1.0.0-alpha.14"
+markdown = "=1.0.0-alpha.17"
 regex = "1"
 semver = "1"
 thiserror = "1"

src/changelog.rs

@@ -210,7 +210,7 @@ pub struct ParseChangelogError(#[from] ParseChangelogErrorInternal);
 #[derive(Debug, Error)]
 enum ParseChangelogErrorInternal {
     #[error("Could not parse changelog as markdown\nError: {0}")]
-    Markdown(String),
+    Markdown(markdown::message::Message),
 
     #[error("Could not parse change group type from changelog - {0}\nError: {1}")]
     InvalidChangeGroup(String, #[source] ParseChangeGroupError),