minor #1340 Clarify purpose of BootableServiceProvider (rbayliss)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Clarify purpose of BootableServiceProvider

`BootableServiceProvider` has an outdated description.  This PR brings the comment into line with the rest of the interfaces in `Silex\Api`

Commits
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4e1ca0c7 Clarify purpose of BootableServiceProvider

src/Silex/Api/BootableProviderInterface.php

@@ -14,7 +14,7 @@ namespace Silex\Api;
 use Silex\Application;
 
 /**
- * Interface that must implement all Silex service providers.
+ * Interface for bootable service providers.
  *
  * @author Fabien Potencier <fabien@symfony.com>
  */