Typescript-mongodb objectIdType Config backwards

https://github.com/dotansimha/graphql-code-generator/blob/6734c58055d283f051bf32f115f2308000e56431/packages/plugins/typescript/mongodb/src/config.ts#L33

This example is backwards from how the config actually imports. Should read:

`config:
objectIdType: MyIdType#./my-models.ts
`

Will open a pull request in a bit to fix. Just wanted to log before I forgot.

Open packages/plugins/typescript/mongodb/src/config.ts at the documented objectIdType example. Compare the example with the import format described in the issue, update the example to use MyIdType#./my-models.ts, and verify that the surrounding configuration documentation remains accurate.

Written by the indexing model from the issue text.