docs: fix up markdown rendering (QwikDev#5532)

packages/docs/src/routes/docs/(qwikcity)/guides/mdx/index.mdx

@@ -33,7 +33,7 @@ src/
             └── index.mdx    # https://example.com/some/path
 ```
 
-```Markdown title="src/routes/some/path/index.mdx"
+```markdown title="src/routes/some/path/index.mdx"
 ---
 title: Hello World Title
 ---
@@ -59,7 +59,7 @@ src/
             └── index.mdx    # https://example.com/some/path
 ```
 
-```Markdown title="src/routes/some/path/index.mdx"
+```markdown title="src/routes/some/path/index.mdx"
 ---
 title: Hello World Title
 ---
@@ -161,7 +161,7 @@ The above example will generate the following HTML code.
 ## Reading frontmatter data
 Frontmatter keys are accessible by leveraging the `useDocumentHead()` hook.
 
-```Markdown title="src/routes/some/path/index.mdx"
+```markdown title="src/routes/some/path/index.mdx"
 ---
 title: Hello World Title
 tags: