usb: gadget: Fix typo fond in Documentation/Docbook/gadget.xml

This patch fix some spelling typo found in gadget.xml.
It is because this file is generated from comments in sources,
I had to fix comments in the source, instead of xml file itself.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>