fix formula

[futantan]

trafficstars

Add a space so that the formula can be parsed

What type of PR is this (这是什么类型的PR)

• [x] Typo(勘误)

Which issue(s) this PR fixes(Optional) (这个PR 修复了什么问题 (可选择))

• resolve(修复) : #

What this PR does / why we need it (这个PR 做了什么/ 我们为什么需要这个PR)

[futantan]

Before: After:

[AmazingAng]

lgtm