[MonologBundle] Added token to XSD

When using the LogEntriesHandler combined with XML configuration, the config file will not validate since the 'token' attribute is missing from the XSD
 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |