Fix position operator documentation

[utkarshr3144]

Summary

Fix an incorrect documentation heading in Position.lean.

The section describes generalized eigenvectors of positionOperatorUnbounded, but the heading incorrectly referred to the momentum operator. This changes the heading to correctly refer to the position operator.

Verification

• git diff --check

Fixes #1540

[github-actions]

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[jstoobysmith]

Approved - many thanks! Will merge shortly 🙂