binman: Add a missing comment in Entry_vblock

An important property is missing. Update the entry comment to include
this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/binman/etype/vblock.py b/tools/binman/etype/vblock.py
index c4d970e..334ff9f 100644
--- a/tools/binman/etype/vblock.py
+++ b/tools/binman/etype/vblock.py
@@ -18,6 +18,7 @@
     """An entry which contains a Chromium OS verified boot block
 
     Properties / Entry arguments:
+        - content: List of phandles to entries to sign
         - keydir: Directory containing the public keys to use
         - keyblock: Name of the key file to use (inside keydir)
         - signprivate: Name of provide key file to use (inside keydir)