Fix spelling of thin-manifests option.

Currently not causing any issues due to use-manifests being in lenient mode,
but should sort this prior.

BUG=chromium-os:11308
TEST=repoman manifest a pkg, check the results

Change-Id: I8006fb04ba84cef372881d6d684b35fe99d30f06
Reviewed-on: http://gerrit.chromium.org/gerrit/10122
Reviewed-by: David James <davidjames@chromium.org>
Commit-Ready: Brian Harring <ferringb@chromium.org>
Reviewed-by: Brian Harring <ferringb@chromium.org>
Tested-by: Brian Harring <ferringb@chromium.org>
diff --git a/metadata/layout.conf b/metadata/layout.conf
index 854b0be..63c9d88 100644
--- a/metadata/layout.conf
+++ b/metadata/layout.conf
@@ -1,3 +1,3 @@
 masters = gentoo portage-stable chromiumos
 use-manifests = true
-thin-manifest = true
+thin-manifests = true