Corrected plugin documentations based on Issue #6752 (#6798) (#6858)

Co-authored-by: Maximilian Konrad <[email protected]>

Author: FinalAngel

CHANGELOG.rst

@@ -49,6 +49,7 @@ Changelog
 * Improved documentation related to nested plugins.
 * Updated installation tutorial.
 * Fixed a simple typo in the docstring for ``cms.utils.helpers.normalize_name``.
+* Updated 'How to create Plugins' Tutorial.
 
 
 3.7.0 (2019-09-25)

docs/how_to/custom_plugins.rst

@@ -263,7 +263,7 @@ it becomes::
         sections = models.ManyToManyField(Section)
 
         def copy_relations(self, oldinstance):
-            self.sections = oldinstance.sections.all()
+            self.sections.set(oldinstance.sections.all())
 
 If your plugins have relational fields of both kinds, you may of course need
 to use *both* the copying techniques described above.