DOC fix kwargs parameter name in curvefit docstrings

[advitrocks9]

Description

Fixes #6891.

Dataset.curvefit and DataArray.curvefit both document their scipy passthrough as
**kwargs : optional, so calling them the way the docstring describes fails:

import numpy as np
import xarray as xr

def exp(t, a, b):
    return a * np.exp(b * t)

t = np.linspace(0, 1, 20)
da = xr.DataArray(exp(t, 2.0, -1.0), dims="t", coords={"t": t})
da.curvefit("t", exp, method="trf")

TypeError: DataArray.curvefit() got an unexpected keyword argument 'method'

Same on Dataset. Neither method actually has a VAR_KEYWORD parameter, they take a single kwargs: dict[str, Any] | None = None, and kwargs={"method": "trf"} works fine. So the docstrings are wrong, not the behaviour. computation/fit.py,
which both methods delegate to, already documents the parameter correctly, the class docstrings
just drifted from it in #5182. headtr1ck
scoped this fix on the issue:
change **kwargs to kwargs in both docstrings.

Checklist

• Closes Passing extra keyword arguments to curvefit throws an exception. #6891
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst