[BUGFIX] Wrong CSS definition for "submit"

Exchange the CSS style definition for the non-existant element "submit"
with "button", which is supposed to be meant.

Change-Id: Ib865addda8e3729970a4b72c1eb233e257ae9074
Resolves: #M18043
Reviewed-on: http://review.typo3.org/1279
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert