Fix bug in GraphMatch docs

[bdpedigo]

• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
  • Is the change to the API backwards compatible?
• Have you built the documentation (reference and/or tutorial) and verified the generated documentation is appropriate?

Reference Issues/PRs

None

What does this implement/fix? Briefly explain your changes.

Fixes a bug in the GraphMatch docstring about initializations from the barycenter

Any other comments?

@asaadeldin11 this is no longer true at all, right?

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[asaadeldin11]

Yeah, no longer do this

[bdpedigo]

@asaadeldin11 just noticed something else while I'm here
Under init docstring

If 2d-array, init must be :math:m' x m', where :math:m' = n - m,
and it must be doubly stochastic: each of its rows and columns must
sum to 1.

I don't actually know what m' is? I assume the number of nonseeded vertices? I don't think we actually define there