Add missing underscore to fix rendering

[frankroeder]

Description

Rendering the fetch environment raises an error, that get_gripper_xpos is not defined.
Looking into the code, I noticed that the leading underscore is missing, changing it from self.get_gripper_xpos to self._get_gripper_xpos resolved the issue.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[rodrigodelazcano]

Thanks! LGTM