fastboot: document second parameter of 'fetch'

Add the OUT_FILE parameter to the documentation of fetch which fails if
used as currently (erroneously) documented:

    $ fastboot fetch boot
    fastboot: usage: expected argument

Test: fastboot help
Change-Id: Ia06d40c2f754e1a55a2052cb84e81f96e71b4573
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 6a49fdf..57da2d6 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -429,7 +429,7 @@
             " snapshot-update merge      On devices that support snapshot-based updates, finish\n"
             "                            an in-progress update if it is in the \"merging\"\n"
             "                            phase.\n"
-            " fetch PARTITION            Fetch a partition image from the device."
+            " fetch PARTITION OUT_FILE   Fetch a partition image from the device."
             "\n"
             "boot image:\n"
             " boot KERNEL [RAMDISK [SECOND]]\n"