Add missing title markup. (#9823)

While reading the tutorial doc, I noticed one line that appeared to be missing the title markup.

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docs/tutorial/part-two/index.md

@@ -317,7 +317,7 @@ React in general).
 
 Gatsby works out of the box with CSS Modules.
 
-Build a page using CSS Modules.
+### Build a page using CSS Modules.
 
 First, create a new `Container` component. Create a new directory at
 `src/components` and then, in this new directory, create a file named