Fix assignment to constant

lib/Authorisation.js

@@ -171,7 +171,7 @@ class Authorisation extends EventEmitter {
       const retryAfterHeader = result.headers['Retry-After']
       let pollingDuration = 1000
       if (retryAfterHeader) {
-        retryAfterHeader = parseInt(retryAfterHeader)
+        pollingDuration = parseInt(retryAfterHeader)
       }
 
       log(`   ⌚    ❨auto-encrypt❩ Authorisation not valid yet for domain ${this.domain}. Waiting to check again in ${pollingDuration/1000} second${pollingDuration === 1000 ? '' : 's'}…`)