kbuild: doc: remove outdated description of the limitation on -I usage

Kbuild used to manipulate header search paths, enforcing the odd
limitation of "no space after -I".

Commit cdd750b ("kbuild: remove 'addtree' and 'flags' magic for
header search paths") stopped doing that. This limitation no longer
exists. Instead, you need to accurately specify the header search path.
(In this case, $(src)/include)

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

Author: masahir0y

Documentation/kbuild/modules.rst

@@ -328,13 +328,9 @@ according to the following rule:
 		--> filename: Kbuild
 		obj-m := 8123.o
 
-		ccflags-y := -Iinclude
+		ccflags-y := -I $(src)/include
 		8123-y := 8123_if.o 8123_pci.o 8123_bin.o
 
-	Note that in the assignment there is no space between -I and
-	the path. This is a limitation of kbuild: there must be no
-	space present.
-
 4.3 Several Subdirectories
 --------------------------