docs(contributing): update commit message example (#4560)

Fix #4559

CONTRIBUTING.md

@@ -95,7 +95,7 @@ footer
     - `test` - changes to puppeteer tests infrastructure.
     - `style` - puppeteer code style: spaces/alignment/wrapping etc.
     - `chore` - build-related work, e.g. doclint changes / travis / appveyor.
-2. *namespace* is put in parenthesis after label and is optional.
+2. *namespace* is put in parenthesis after label and is optional. Must be lowercase.
 3. *title* is a brief summary of changes.
 4. *description* is **optional**, new-line separated from title and is in present tense.
 5. *footer* is **optional**, new-line separated from *description* and contains "fixes" / "references" attribution to github issues.
@@ -104,7 +104,7 @@ footer
 Example:
 
 ```
-fix(Page): fix page.pizza method
+fix(page): fix page.pizza method
 
 This patch fixes page.pizza so that it works with iframes.