[FIX] doc: do not duplicate header

And use tabs instead of spaces
Otherwise it looks for the file named "__manifest__.py 2014-..."

doc/howtos/backend/exercise-report

@@ -3,8 +3,8 @@
 
 Index: addons/openacademy/__manifest__.py
 ===================================================================
---- addons.orig/openacademy/__manifest__.py 2014-08-26 17:26:21.535783052 +0200
-+++ addons/openacademy/__manifest__.py  2014-08-26 17:26:21.531783052 +0200
+--- addons.orig/openacademy/__manifest__.py	2014-08-29 08:39:43.975536806 +0200
++++ addons/openacademy/__manifest__.py	2014-08-29 08:39:52.000000000 +0200
 @@ -21,7 +21,7 @@
      'version': '0.1',
 
@@ -14,10 +14,6 @@ Index: addons/openacademy/__manifest__.py
 
      # always loaded
      'data': [
-Index: addons/openacademy/__manifest__.py
-===================================================================
---- addons.orig/openacademy/__manifest__.py	2014-08-29 08:39:43.975536806 +0200
-+++ addons/openacademy/__manifest__.py	2014-08-29 08:39:52.000000000 +0200
 @@ -31,6 +31,7 @@
          'views/openacademy.xml',
          'views/partner.xml',