Fix rendering of yaml example in index.md
Fix rendering of yaml example in index.md

One yaml example is not rendered with yaml syntax highlighting because
the `yaml` type is misspelled as `yamll`.

Change-Id: I9e2a5c2456013884ae121f8bea82ccc16ebc5ee0

tomjohnson1492/documentation-theme-jekyll,tomjohnson1492/documentation-theme-jekyll,tomjohnson1492/documentation-theme-jekyll,tomjohnson1492/documentation-theme-jekyll