fix type-o in README

README.md

@@ -14,7 +14,7 @@ Usage: smtp2http [-v|--verbose] [-s|--silent|-q|--quiet]
  -v --verbose           Log information to STDOUT
 
 TLS Option
-The --tls option accepts a colon-delimited list of certificate file.
+The --tls option accepts a colon-delimited list of certificate files.
 You can specify a single combined PFX file, a cert file followed by a
 key file, or a cert file followed by a key file followed by a signing
 authority certificate.
@@ -89,4 +89,4 @@ openssl pkcs12 -export \
     -out example.com.pfx \
     -in example.com.crt -inkey example.com.key \
     -certfile example.com-ca.crt
-```
+```