Fix @vercel/og license SPDX expression (#74745)
### What?
A minor fix to the `package.json` for `@vercel/og`; there is a typo on
the SPDX license ID for MPL-2.0 which is causing the license ID to not
be correctly extracted by some automated tooling (one example is
https://github.com/CycloneDX/cyclonedx-webpack-plugin, meaning that the
license ID isn't correctly identified).
### Why?
For apps that use automated tooling to generate an exhaustive list of
dependencies, it's useful to have the license identified automatically
and for this to be a valid SPDX identifier to be able to group
dependencies together.
Co-authored-by: Jiachi Liu <inbox@huozhi.im>
