Fix create_cue_text_leaf_node passing in WEBVTT_TIME_STAMP as kind.

The function webvtt_create_cue_text_leaf_node was passing in the kind WEBVTT_TIME_STAMP
instead of WEBVTT_TEXT kind.
Merge pull request #200 from RickEyre/create-text-node-fix

Fix create_cue_text_leaf_node passing in WEBVTT_TIME_STAMP as kind.