Fixup aapt usage message

The --preferred-configurations option was renamed to
--preferred-density in fab5087 but only part of the
usage message was changed.

Change-Id: I89d270990023beca19605901d956d29d0b0b848b
diff --git a/tools/aapt/Main.cpp b/tools/aapt/Main.cpp
index 2857b59..4ce4b2c 100644
--- a/tools/aapt/Main.cpp
+++ b/tools/aapt/Main.cpp
@@ -69,7 +69,7 @@
         "        [-S resource-sources [-S resource-sources ...]] \\\n"
         "        [-F apk-file] [-J R-file-dir] \\\n"
         "        [--product product1,product2,...] \\\n"
-        "        [-c CONFIGS] [--preferred-configurations CONFIGS] \\\n"
+        "        [-c CONFIGS] [--preferred-density DENSITY] \\\n"
         "        [--split CONFIGS [--split CONFIGS]] \\\n"
         "        [--feature-of package [--feature-after package]] \\\n"
         "        [raw-files-dir [raw-files-dir] ...] \\\n"