Update trailingSlash parameter name (#17228)
Update trailingSlash parameter name (#17228)

Reflect the change in `trailingSlash` parameter name.

I found this because of the great warning on the build!
```
Warning: The "exportTrailingSlash" option has been renamed to "trailingSlash". Please update your next.config.js.
```
flybayer/next.js,zeit/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js,JeromeFitz/next.js