[DOC beta] Fix CoreObject.toString docs.

Before:

![image](https://www.monosnap.com/image/iUrrLTGeTxrKkYp2RoV6G4sWr.png)

After:

![image](https://www.monosnap.com/image/l8vrebm1oXvx3Bs4HvF4vdkmU.png)