[Bug]: Edit warning message in moltres_xs.py that it works with OpenMC 0.15.2

[smpark7]

Expected behavior

moltres_xs.py shouldn't get an error message when working with OpenMC 0.15.2

Describe the bug

moltres_xs.py prints a warning message that there is no guarantee it works with =>0.14.0

To Reproduce

Steps to reproduce the behavior:
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Screenshots/output

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System Specifications:

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