minor #6810 Fixed the redirection of the previous "performance" book chapter (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
----------

Fixed the redirection of the previous "performance" book chapter

Commits
-------

14419d9 Fixed the redirection of the previous "performance" book chapter

Author: xabbuh

_build/redirection_map

@@ -75,7 +75,7 @@
 /book/from_flat_php_to_symfony2 /introduction/from_flat_php_to_symfony2
 /book/configuration /configuration
 /book/propel /propel/propel
-/book/performance /performance/performance
+/book/performance /performance
 /cookbook/assetic/apply_to_option /assetic/apply_to_option
 /cookbook/assetic/asset_management /assetic/asset_management
 /cookbook/assetic/index /assetic