Add 'jni' to documention for ndk-build

ndk-build is intended to be launched from the application directory,
not the jni directory. Clarify the path in the comments.

Change-Id: Ie0faa89a13d967088a4fd2fd1c647962a9c04217

build/make/Android.mk

@@ -27,7 +27,7 @@
 # Android.mk file in the libvpx directory:
 # LOCAL_PATH := $(call my-dir)
 # include $(CLEAR_VARS)
-# include libvpx/build/make/Android.mk
+# include jni/libvpx/build/make/Android.mk
 #
 # There are currently two TARGET_ARCH_ABI targets for ARM.
 # armeabi and armeabi-v7a.  armeabi-v7a is selected by creating an