Fix typo in art script.

Change-Id: I6a3a04d94fa310147b61da8b2a3617571216c951
diff --git a/tools/art b/tools/art
index d4ab347..d99e2d8 100644
--- a/tools/art
+++ b/tools/art
@@ -464,7 +464,7 @@
 # Extract the dex2oat flags from the list of arguments.
 # -Xcompiler-options arguments are stored in DEX2OAT_FLAGS array
 # -cp argument is split by ':' and stored in DEX2OAT_CLASSPATH
-# -Ximage argument is stored in DEX2OAT_BOOTIMAGE
+# -Ximage argument is stored in DEX2OAT_BOOT_IMAGE
 extract_dex2oat_flags "$@"
 
 # If ANDROID_DATA is the system ANDROID_DATA or is not set, use our own,