chore: add some operations in the example

examples/01_simple_type_checking/01_simple_operations.py

@@ -2,3 +2,10 @@ a: int = 3
 b: int = 4
 
 c = a + b  # -> int
+
+c = "invalid"  # -> can't assign str to int variable
+
+d = True
+e = d + d
+
+f: float = a