manual generation: rename manual-txt into manual-text

The output extension and the generation messages refer to 'text', but the make
target was confusingly 'txt'. This patch changes the make target for
consistency.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>