[Docs] Correct mistakes of examples

[likaixin2000]

Modification

Renamed wrong variables 'f' to 'buffer'.

[CLAassistant]

All committers have signed the CLA.

[zhouzaida]

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[likaixin2000]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.You have signed the CLA already but the status is still pending? Let us recheck it.

Hi, thanks for your contribution. Please sign the CLA.

CLA signed.

[OpenMMLab-Assistant005]

Hi @Happylkx ！First of all, we want to express our gratitude for your significant PR in the MMCV project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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