[3.2.x] Fixed #18549 -- Fixed heading for inlines with a OneToOneField.

Used verbose_name instead of verbose_name_plural.

Backport of 0aff3fd711 from master.

tests/admin_inlines/models.py

@@ -44,6 +44,7 @@ class Book(models.Model):
 class Author(models.Model):
     name = models.CharField(max_length=50)
     books = models.ManyToManyField(Book)
+    person = models.OneToOneField('Person', models.CASCADE, null=True)
 
 
 class NonAutoPKBook(models.Model):