Complete a comment about `TARGET_FLATTEN_APEX` and the ART APEX in Android.mk.

Test: n/a
Bug: 132413565
Change-Id: Icd4926c8eed7d8f535d98a93ce31cf3102d6ad77
diff --git a/Android.mk b/Android.mk
index 65a5741..2008c7f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -381,7 +381,7 @@
 # file which is the file that is guaranteed to be created regardless of the
 # value of TARGET_FLATTEN_APEX.
 #
-# b/132413565: Also, when TARGET_FLATTEN_APEX, an empty directory
+# b/132413565: Also, when TARGET_FLATTEN_APEX is true, an empty directory
 # /system/apex/com.android.art is created. After the entire
 # /system/apex is mounted on /apex, the flattened ART APEX
 # (either com.android.art.debug or *.release) is mounted on the empty