docs/manual: fix FOO_PKGDIR name and definition

Change "foo" references to "libfoo" in section that defines variables
explicitly stated to relate to a "libfoo" package.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -528,8 +528,8 @@ endef
 
 In the action definitions, you can use the following variables:
 
-* +$(FOO_PKGDIR)+ contains the path to the directory containing the
-  +foo.mk+ and +Config.in+ files. This variable is useful when it is
+* +$(LIBFOO_PKGDIR)+ contains the path to the directory containing the
+  +libfoo.mk+ and +Config.in+ files. This variable is useful when it is
   necessary to install a file bundled in Buildroot, like a runtime
   configuration file, a splashscreen image...