minor symfony#20625 [FrameworkBundle] Add missing example for 'path' argument in debug:config (chalasr)

This PR was merged into the 3.2 branch.

Discussion
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[FrameworkBundle] Add missing example for 'path' argument in debug:config

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The argument has been introduced in symfony#18940 .

Commits
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c6b7aeb Add missing example for 'path' argument in debug:config

Author: nicolas-grekas

src/Symfony/Bundle/FrameworkBundle/Command/ConfigDebugCommand.php

@@ -47,6 +47,10 @@ protected function configure()
   <info>php %command.full_name% framework</info>
   <info>php %command.full_name% FrameworkBundle</info>
 
+For dumping a specific option, add its path as second argument:
+
+  <info>php %command.full_name% framework serializer.enabled</info>
+
 EOF
             )
         ;