Fix typo in declarations.md
Otherwise this would error with:
```
Polyaxonfile is not valid.
Error message `'batch' is undefined`.
```
@@ -49,7 +49,7 @@ declarations:
This declaration can be used to pass values to our program:
```yaml
- ... --batch-size={{ batch-size }}
+ ... --batch-size={{ batch_size }}
```
Or
```yaml
