Fixed typo (openhab#4956)

Changed 'forth' to 'fourth'.

Signed-off-by: Pshatsillo <[email protected]>

addons/binding/org.openhab.binding.mqtt/README.md

@@ -75,7 +75,7 @@ A third connection uses a username and password for authentication.
 Secure is set to false as the username and password is requested by the broker.
 The credentials are plain values on the wire, therefore you should only use this on a secure connection.
 
-In a forth connection, the public key pinning is enabled again.
+In a fourth connection, the public key pinning is enabled again.
 This time, a public key hash is provided to pin the connection to a specific server.
 It follows the form "hashname:hashvalue". Valid *hashnames* are SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 and all others listed
 in [Java MessageDigest Algorithms](https://docs.oracle.com/javase/9/docs/specs/security/standard-names.html#messagedigest-algorithms).