Correct 'thing' to users on manage folder page

The checkboxes component that lets you choose
which users have permission to see the folder is
currently using the noun of 'folder' in the
summary it generates.

This is because this page users the
TemplateFolderForm class, which defaults the
permissions to 'folder'.

This updates the field_label attribute, used by
the code that generates the summary, to 'user' to
match the type of permission.

app/main/forms.py

@@ -2126,7 +2126,7 @@ class TemplateFolderForm(StripWhitespaceForm):
 
     users_with_permission = GovukCollapsibleCheckboxesField(
         'Team members who can see this folder',
-        field_label='folder')
+        field_label='team member')
     name = GovukTextInputField('Folder name', validators=[DataRequired(message='Cannot be empty')])