[FIX] doc: correct patch line number

to avoid "WARNING: Hunk"

Closes #18286

doc/howtos/backend/exercise-calendar

@@ -11,7 +11,7 @@ diff --git a/openacademy/models.py b/openacademy/models.py
 +from datetime import timedelta
  from odoo import models, fields, api, exceptions
  
- class Session(models.Model):
+ class Course(models.Model):
 @@ -55,6 +56,8 @@ class Session(models.Model):
      attendee_ids = fields.Many2many('res.partner', string="Attendees")
  

doc/howtos/backend/exercise-kanban

@@ -17,7 +17,7 @@ Index: addons/openacademy/views/openacademy.xml
 ===================================================================
 --- addons.orig/openacademy/views/openacademy.xml	2014-08-28 14:21:58.627015606 +0200
 +++ addons/openacademy/views/openacademy.xml	2014-08-28 14:21:58.623015606 +0200
-@@ -169,11 +169,57 @@
+@@ -168,11 +168,57 @@
              </field>
          </record>