Fix typo for max-files

In XML a dash is used for the attribute max-files, but in the comment above that XML element an underscore was used (max_files).
Other XML examples in the Symfony Docs (like the routing doc page) use dashes for XML attributes, so I concluded that this XML attribute is also correctly written, and the comment has to be changed instead.

Author: vmeijer

logging.rst

@@ -315,7 +315,7 @@ option of your handler to ``rotating_file``:
                 https://symfony.com/schema/dic/monolog/monolog-1.0.xsd">
 
             <monolog:config>
-                <!-- "max_files": max number of log files to keep
+                <!-- "max-files": max number of log files to keep
                      defaults to zero, which means infinite files -->
                 <monolog:handler name="main"
                     type="rotating_file"