[Doc] Fix missing quote in tutorial/step4

[TurtleSmoke]

There is an error in the code of the example file axelor-contact/src/main/resources/views/Contact.xml in step 4 of the Quick Tutorial.

The error, which is a missing quote, causes the XML to be invalid.

This error can be found in the following versions of the tutorial:

• 5.4
• 6.0
• 6.1