UML OCP finished, Start with Liskov Substitution Principle

Intro_to_Solid.md

@@ -144,7 +144,7 @@ class InvoicePersistence(abc.ABC):
 The new _InvoicePersistence_ class has an abstract method.
 So, if a class inherits the _InvoicePersistence_ class, you have to implement the _save_ method.
 
-And for example, we will create a _DataBasePersistence_ class, and this class inherit the abstract _InvoicePersistence_ class.
+And for example, we will create a _DataBasePersistence_ class, and this class inherits the abstract _InvoicePersistence_ class.
 ```python
 class DatabasePersistence(InvoicePersistence):
     def __init__(self, invoice):
@@ -154,7 +154,7 @@ class DatabasePersistence(InvoicePersistence):
     def save(self, invoice):
         print("Save in database ...")
 ```
-Let's make the same thing with _FilePersistence_ class.
+Let's do the same thing with _FilePersistence_ class.
 ```python
 class FilePersistence(InvoicePersistence):
     def __init__(self, invoice):
@@ -164,3 +164,13 @@ class FilePersistence(InvoicePersistence):
     def save(self, invoice):
         print("Save to file ...")
 ```
+Ok, we do a lot of things, let's make a UML to represent our class structure.
+
+<center>
+<img src="./assets/UML_Solid" alt=""  width="400"/>
+</center>
+
+That will be more simple to do extension.
+
+### Liskov Substitution Principle
+