doc/manuals/Makefile: fix typo in 'srcdir'

Prevent the following error (generating manuals seemed to work fine
regardless):
	realpath: missing operand
	Try 'realpath --help' for more information.
	make: Nothing to be done for 'all'.

Change-Id: I72a62b86cba0dfd710f7e1c4c01a3c10710128d8
1 file changed