Extra apostrophe at the beginning of PROTOCOL.agent

[HARUYAMA Seigo]

diff --git a/PROTOCOL.agent b/PROTOCOL.agent
index 67302c34..81ae8cfd 100644
--- a/PROTOCOL.agent
+++ b/PROTOCOL.agent
@@ -1,7 +1,7 @@
 The SSH agent protocol is described in
 https://tools.ietf.org/html/draft-miller-ssh-agent-04
 
-This file document's OpenSSH's extensions to the agent protocol.
+This file documents OpenSSH's extensions to the agent protocol.
 
 1. session-bind at openssh.com extension