docs: fix MultiMixEnv docstring referencing step instead of reset

[william-xue]

Summary

The MultiMixEnv class docstring incorrectly states that a different mix is used each time the step function is called. In practice, the mix rotates on reset, not step — each step call continues within the same mix until the episode ends and reset is called again.

This one-word fix corrects the docstring from "step" to "reset" to match the actual behavior.

Test plan

• Verify the MultiMixEnv class docstring now says "reset" instead of "step"
• Confirm the behavior matches: calling reset() cycles to the next mix, while step() stays in the current mix

Fixes #734

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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[BDonnot]

Hello,

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