Fix typos in bidi-img-alt-text, s/rtr/rtl/

Fix a couple of types in LayoutTests/fast/text/bidi-img-alt-text.html
where it said dir="rtr" instead of dir="rtl" thus not actually testing
the behavior in the rtl case.

BUG=326921
TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/115723007

git-svn-id: svn://svn.chromium.org/blink/trunk@164457 bbb929c8-8fbe-4397-9dbb-9b2b20218538