Update AGENT.md

Fixing typo

Author: dcarbone

docs/AGENT.md

@@ -115,7 +115,7 @@ if (null !== $err)
 ### Deregister Agent Check
 
 ```php
-$err = $client->Agetn()->checkDeregister('dan test service check');
+$err = $client->Agent()->checkDeregister('dan test service check');
 if (null !== $err)
     die($err);
 ```
@@ -190,4 +190,4 @@ if (null !== $err)
 $err = $client->Agent->failTTL('check id', 'super not good here, dudes.');
 if (null !== $err)
     die($err);
-```
+```