Change subfile for constraint energy observation

[knelli2]

Proposed changes

Forgot that I have to change the subfile name, otherwise our H5 Dat file will complain

Upgrade instructions

Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments