Fix build system documentation example

The input and output file paths were mixed up in this example. Properly
indicate the input file as a dependency.

Change-Id: I71271025c6ce3ef8f82b10aee91fc0c7ae8f2399
diff --git a/core/build-system.html b/core/build-system.html
index bddde6a..95f35ce 100644
--- a/core/build-system.html
+++ b/core/build-system.html
@@ -438,7 +438,7 @@
 GEN := $(intermediates)/<font color=red>file.c</font>
 $(GEN): PRIVATE_INPUT_FILE := $(LOCAL_PATH)/<font color=red>input.file</font>
 $(GEN): PRIVATE_CUSTOM_TOOL = <font color=red>cat $(PRIVATE_INPUT_FILE) &gt; $@</font>
-$(GEN): <font color=red>$(LOCAL_PATH)/file.c</font>
+$(GEN): <font color=red>$(LOCAL_PATH)/input.file</font>
 	$(transform-generated-source)
 LOCAL_GENERATED_SOURCES += $(GEN)
 </pre>