Remove "anomaly" language from WFI18 transient step documentation

[schlafly]

There is sensitivity around the word "anomaly" in connection with wfi18. We ended up naming the step wfi18_transient, which is great. But some of the associated documentation uses the word anomaly. We need to update the documentation to remove the work anomaly, instead preferring only "transient." e.g., the title of this page should become WFI18 transient correction instead of WFI18 transient anomaly correction.
https://roman-pipeline.readthedocs.io/en/stable/roman/wfi18_transient/index.html

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