kbuild: two minor updates for Documentation/kbuild/modules.rst

Capitalize the first word in the sentence.

Use obj-m instead of obj-y. obj-y still works, but we have no built-in
objects in external module builds. So, obj-m is better IMHO.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/modules.rst

@@ -502,7 +502,7 @@ build.
 ----------------------------------------
 
 	Sometimes, an external module uses exported symbols from
-	another external module. kbuild needs to have full knowledge of
+	another external module. Kbuild needs to have full knowledge of
 	all symbols to avoid spitting out warnings about undefined
 	symbols. Three solutions exist for this situation.
 
@@ -522,7 +522,7 @@ build.
 		The top-level kbuild file would then look like::
 
 			#./Kbuild (or ./Makefile):
-				obj-y := foo/ bar/
+				obj-m := foo/ bar/
 
 		And executing::