BackupHelpers: Fix Typo

delete --> free

Change-Id: Idbf7e677f8cc4a9a9d04e58b5cf7d0f1589c5211
diff --git a/libs/androidfw/BackupHelpers.cpp b/libs/androidfw/BackupHelpers.cpp
index 7a817a7..dcf41b7 100644
--- a/libs/androidfw/BackupHelpers.cpp
+++ b/libs/androidfw/BackupHelpers.cpp
@@ -710,7 +710,7 @@
     }
 
 cleanup:
-    delete [] buf;
+    free(buf);
 done:
     close(fd);
     return err;