kbuild: fix documentation in makefiles.txt

Small error had sneaked in with respect to use
of LDFLAGS_$@. LDFLAGS_$@ is not valid in normal
kbuild files so do not say so.
Fix a reference bug too.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/makefiles.txt

@@ -780,8 +780,8 @@ When kbuild executes, the following steps are followed (roughly):
 	Example:
 		#arch/s390/Makefile
 		LDFLAGS         := -m elf_s390
-	Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
-	the flags used. See chapter 7.
+	Note: EXTRA_LDFLAGS can be used to further customise
+	the flags used. See chapter 3.7.
 
     LDFLAGS_MODULE	Options for $(LD) when linking modules