Fixes minor typo in documentation

[LukasOpp]

trafficstars

Changes dataloaser -> dataloader

[CLAassistant]

All committers have signed the CLA.

[MeowZheng]

Many thanks for your nice pr, please sign CLA

[MeowZheng]

Please sign the Contributor License Agreement

[OpenMMLab-Assistant-004]

Hi @LukasOpp ！We are grateful for your efforts in helping improve mmsegmentation open-source project during your personal time.

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