Fix for basic-tutorial-7.c (Simple multithreaded example)

[Charlie Turner]

I'm not sure where basic-tutorial-7.c lives, the tutorial itself says to
find it in my GStreamer installation, but I can't, nor in the git repo.

Anyway, I found I had to make this change to get it to run in 1.12.2,
perhaps a note could be added to the tut?

- tee_src_pad_template = gst_element_class_get_pad_template
(GST_ELEMENT_GET_CLASS (tee), "src_%d");
+ tee_src_pad_template = gst_element_class_get_pad_template
(GST_ELEMENT_GET_CLASS (tee), "src_%u");

(note it's s/src_%d/src_%u/)

BR,
--Charlie.