Fix step activation docstring to match >= threshold behavior

[ayaangazali]

Proposed changes

The step function and Step module docstrings say the output is "1 if the input is greater than a specified threshold", but the implementation is mx.where(x >= threshold, 1, 0) and the math block right below the prose already uses x >= threshold. So the prose was the only part that disagreed. Quick check on main:

import mlx.core as mx
import mlx.nn as nn

nn.Step(0.0)(mx.array([-1.0, 0.0, 1.0])).tolist()  # [0, 1, 1]  -> 0 maps to 1, i.e. >=

Updated the prose to "greater than or equal to" in both places, and documented the threshold default (0.0) while I was there.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

(docstring-only, test_nn.py still passes 69/69)

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quick note: i'm a freshman still learning, and i lean on Claude Code to surface these doc/code mismatches, but i ran the snippet above myself to confirm the boundary value maps to 1 before writing this. happy to adjust if you'd word it differently.