Update docs/Learning-Environment-Design-Agents.md

Co-authored-by: andrewcoh <54679309+andrewcoh@users.noreply.github.com>

docs/Learning-Environment-Design-Agents.md

 its GameObject.
 The `BufferSensor` can be useful in situations in which the Agent must pay
 attention to a varying number of entities. On the trainer side, the `BufferSensor`
-is processed using an attention module, more information about attention
+is processed using an attention module. More information about attention
 mechanisms can be found [here](https://arxiv.org/abs/1706.03762). Training or
 doing inference with variable length observations can be slower than using
 a flat vector observation, but can allow you to represent more complex problems