[CLEANUP] Typo in INSTALL.txt

Change-Id: I2597cbec82e2dc61ab247cdff8b4ce9d19b2c231
Reviewed-on: http://review.typo3.org/12942
Reviewed-by: Wouter Wolters
Reviewed-by: Oliver Klee
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert