corrected typo in instructions for "run the server"

Keygen instructions suggestion using the folder named "cert".
Server run instructions use the folder "certs".
This change harmonizes the two to be "cert".

README.md

@@ -56,8 +56,8 @@ You're ready to run the server:
 
     $ ./bin/redoctober -addr=localhost:8080 \
                        -vaultpath=diskrecord.json \
-                       -cert=certs/server.crt \
-                       -key=certs/server.pem \
+                       -cert=cert/server.crt \
+                       -key=cert/server.pem \
                        -static=$GOPATH/src/github.com/cloudflare/redoctober/index.html
 
 ## Quick start: example webapp