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
diff --git a/doc/manuals/Makefile b/doc/manuals/Makefile
index d9a613d..2f87b57 100644
--- a/doc/manuals/Makefile
+++ b/doc/manuals/Makefile
@@ -1,6 +1,6 @@
 OSMO_GSM_MANUALS_DIR:=$(shell ./osmo-gsm-manuals-dir.sh)
 
-sdcdir = .
+srcdir = .
 
 ASCIIDOC = icE1usb-usermanual.adoc
 include $(OSMO_GSM_MANUALS_DIR)/build/Makefile.asciidoc.inc