docs: fix typo

Author: rpenido

xblock/core.py

@@ -687,7 +687,7 @@ class XBlock(Plugin, Blocklike, metaclass=_HasChildrenMetaclass):
 
     # These are dynamically managed by the awful hackery of _HasChildrenMetaclass.
     # We just declare their types here to make static analyzers happy.
-    # Note that children is only defined iff has_children is defined and True.
+    # Note that children is only defined if has_children is defined and True.
     has_children: bool
     children: ReferenceList