fix typo in log message (#3987)

com.unity.ml-agents/Runtime/Sensors/SensorShapeValidator.cs

                     Debug.Assert(cachedShape.Length == sensorShape.Length, "Sensor dimensions must match.");
                     for (var j = 0; j < Mathf.Min(cachedShape.Length, sensorShape.Length); j++)
                     {
-                        Debug.Assert(cachedShape[j] == sensorShape[j], "Sensor sizes much match.");
+                        Debug.Assert(cachedShape[j] == sensorShape[j], "Sensor sizes must match.");
                     }
                 }
             }

com.unity.ml-agents/Tests/Editor/Sensor/SensorShapeValidatorTests.cs

             validator.ValidateSensors(sensorList1);
 
             var sensorList2 = new List<ISensor>() { new DummySensor(1), new DummySensor(2, 3), new DummySensor(4, 5, 7) };
-            LogAssert.Expect(LogType.Assert, "Sensor sizes much match.");
+            LogAssert.Expect(LogType.Assert, "Sensor sizes must match.");
-            LogAssert.Expect(LogType.Assert, "Sensor sizes much match.");
+            LogAssert.Expect(LogType.Assert, "Sensor sizes must match.");
             validator.ValidateSensors(sensorList1);
         }
 
             var sensorList2 = new List<ISensor>() { new DummySensor(1), new DummySensor(9) };
             LogAssert.Expect(LogType.Assert, "Number of Sensors must match. 3 != 2");
             LogAssert.Expect(LogType.Assert, "Sensor dimensions must match.");
-            LogAssert.Expect(LogType.Assert, "Sensor sizes much match.");
+            LogAssert.Expect(LogType.Assert, "Sensor sizes must match.");
             validator.ValidateSensors(sensorList2);
 
             // Add the sensors in the other order
             LogAssert.Expect(LogType.Assert, "Sensor dimensions must match.");
-            LogAssert.Expect(LogType.Assert, "Sensor sizes much match.");
+            LogAssert.Expect(LogType.Assert, "Sensor sizes must match.");
             validator.ValidateSensors(sensorList1);
         }
     }