Masking Discrete Actions typos (#4961)

docs/Learning-Environment-Design-Agents.md

 ```csharp
 public override void WriteDiscreteActionMask(IDiscreteActionMask actionMask)
 {
-    actionMasker.WriteMask(branch, actionIndices)
+    actionMask.WriteMask(branch, actionIndices);
 }
 ```
 
 are masked)
 
 ```csharp
-WriteMask(0, new int[2]{1,2})
+WriteMask(0, new int[2]{1,2});
 ```
 
 Notes: