Allow admonition blockquotes with blank lines

This fixes an issue where if an admonition blockquote had multiple
paragraphs, it would incorrectly interpret it as multiple blockquotes,
and break the formatting. The problem is that the regex was expecting at
least one space after the `>`.

I vaguely recall doing this intentionally, but I don't remember now why
I did it this way, and I can't find any problems with this approach.

Author: ehuss

mdbook-spec/src/lib.rs

@@ -19,7 +19,7 @@ static RULE_RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(?m)^r\[([^]]+)]$").unwr
 /// The Regex for the syntax for blockquotes that have a specific CSS class,
 /// like `> [!WARNING]`.
 static ADMONITION_RE: Lazy<Regex> = Lazy::new(|| {
-    Regex::new(r"(?m)^ *> \[!(?<admon>[^]]+)\]\n(?<blockquote>(?: *> .*\n)+)").unwrap()
+    Regex::new(r"(?m)^ *> \[!(?<admon>[^]]+)\]\n(?<blockquote>(?: *>.*\n)+)").unwrap()
 });
 
 pub fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<(), Error> {