[BuildPage] Fix a typo in contribution.md
[BuildPage] Fix a typo in contribution.md

Change-Id: Ia644ba0ece93c6d7473d2a791070dc7c3a8f4ce7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2383401
Auto-Submit: Quang Minh Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Reviewed-by: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go