typo: mistake with async function signature in README.md

[Kamikadze4GAME]

Fixed small mistake with async function signature in README.md

The line

migrate-mongo/README.md

Line 132 in 4906ff2

* `function async up(db, client) { /* */ }` should contain `await` keyword(s) and return `Promise`

should be changed into

* `async function up(db, client) { /* */ }` should contain `await` keyword(s) and return `Promise`

Checklist

• README.md is updated

[coveralls]

Coverage remained the same at 100.0% when pulling f3cd105 on Kamikadze4GAME:patch-1 into 4906ff2 on seppevs:master.