Update CatalogItem.cs

A small spelling mistake.

src/Services/Catalog/Catalog.API/Model/CatalogItem.cs

@@ -64,7 +64,7 @@ namespace Microsoft.eShopOnContainers.Services.Catalog.API.Model
 
             if (quantityDesired <= 0)
             {
-                throw new CatalogDomainException($"Item units desired should be greater than cero");
+                throw new CatalogDomainException($"Item units desired should be greater than zero");
             }
 
             int removed = Math.Min(quantityDesired, this.AvailableStock);
@@ -100,4 +100,4 @@ namespace Microsoft.eShopOnContainers.Services.Catalog.API.Model
             return this.AvailableStock - original;
         }
     }
-}
+}