README: fix gyp command line

Change-Id: Id51e77b7ac193fd777a43c1fd0943708a112eeb3
Reviewed-on: https://weave-review.googlesource.com/1324
Reviewed-by: Vitaly Buka <vitalybuka@google.com>

libweave/README

diff --git a/libweave/README b/libweave/README
index def7148..d8fbd3f 100644
--- a/libweave/README
+++ b/libweave/README
@@ -91,7 +91,7 @@
 
 Generate ninja build files:
 
-  gyp -I libweave_common.gypi --toplevel-dir=. \
+  gyp -I libweave_common.gypi --toplevel-dir=. --depth=. \
       -f ninja libweave_standalone.gyp
 
 Build library with tests: