Fix MACM analysis example

[JulioAPeraza]

Closes None.

Changes proposed in this pull request:

• I think atlas["maps"] is already a nibabel.nifti1.Nifti1Image object. Using nib.load on atlas["maps"] will raise the error:

TypeError: stat: path should be string, bytes, os.PathLike or integer, not Nifti1Image

[codecov]

Codecov Report

Base: 88.55% // Head: 88.05% // Decreases project coverage by -0.50% ⚠️

Coverage data is based on head (6f83304) compared to base (e269941).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #750      +/-   ##
==========================================
- Coverage   88.55%   88.05%   -0.51%     
==========================================
  Files          38       35       -3     
  Lines        4370     3884     -486     
==========================================
- Hits         3870     3420     -450     
+ Misses        500      464      -36     

Impacted Files	Coverage Δ
nimare/__init__.py
nimare/utils.py
nimare/base.py

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[jdkent]

Looks correct, checked the oldest version of nilearn that NiMARE supports, and it looks like it should be a nifti image and not a filename:
https://github.com/nilearn/nilearn/blob/ad04ee8ce91fde193fe4627f666ddeb52271a383/nilearn/datasets/atlas.py#L271-L429