Removed wrong reference to cookbook

The /cookbook/request/mime_type article does not contain an example to
register a kernel listener since 2.5
Merge branch 'franckduche-patch-1' into 2.4
Merging #545
Merge branch '2.5' into 2.4
Update introduction.rst

fix spelling
Merge pull request #559 from symfony-fr/2.5

Merging 2.5 to master
Merge pull request #555 from CoreFloDev/patch-1

Update introduction.rst
Merge pull request #548 from gquemener/remove_incorrect_reference

Removed wrong reference to cookbook
Merge branch '2.5' into 2.4
Merging #552