minor #7192 adding the namespace for ArrayInput (nodefortytwo)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #7192).

Discussion
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adding the namespace for ArrayInput

Makes the documentation a little cleaner, easier to copy

Commits
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9e04c4c adding the namespace for ArrayInput

Author: xabbuh

console/calling_commands.rst

@@ -10,6 +10,9 @@ generating Doctrine2 proxies, dumping Assetic assets, ...).
 
 Calling a command from another one is straightforward::
 
+    use Symfony\Component\Console\Input\ArrayInput;
+    // ...
+
     protected function execute(InputInterface $input, OutputInterface $output)
     {
         $command = $this->getApplication()->find('demo:greet');