Small Tov documentation typos

[pajkosmi]

Proposed changes

In the TovStar documentation, this PR fixes an exponent for the expression of the radial metric potential (see Eq. 26.37 of Thorne & Blandford) and the expression of the spatial metric in Cartesian coordinates in terms of the angular metric potential, to match the calculations in TovStar.cpp.

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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.

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