correct plural for PpiFamily class

Former-commit-id: 0e507767070425a3f0686d7c6c1dc973aef9b5d3

ippisite/ippidb/models.py

@@ -204,6 +204,9 @@ class Disease(models.Model):
 class PpiFamily(models.Model):
     name = models.CharField('Name', max_length=30, unique=True)
 
+    class Meta:
+        verbose_name_plural = "PPI Families"
+
     def __str__(self):
         return self.name