fix: fix usage of wrong regex match group

The current regex match group we're using to get the mermaid diagrams
from markdown prepends a `\n` character to every mermaid diagram.

This only currently works because the current
mmdc -> HTML -> mermaid.init step removes these newlines automatically.

src/index.js

@@ -449,7 +449,7 @@ async function run (input, output, { puppeteerConfig = {}, quiet = false, output
     if (input && /\.(md|markdown)$/.test(input)) {
       const imagePromises = []
       for (const mermaidCodeblockMatch of definition.matchAll(mermaidChartsInMarkdownRegexGlobal)) {
-        const mermaidDefinition = mermaidCodeblockMatch[1]
+        const mermaidDefinition = mermaidCodeblockMatch[2]
 
         /** Output can be either a template image file, or a `.md` output file.
          *   If it is a template image file, use that to created numbered diagrams