corrects blunder in schema.mdx

[xpanvictor]

changes an List type to a List type.

Closes #1933

Description

Just changes text an List type to a List type.

[vercel]

@xpanvictor is attempting to deploy a commit to the The GraphQL Foundation Team on Vercel.

A member of the Team first needs to authorize it.

[linux-foundation-easycla]

• ❌ login: @xpanvictor / The commit (54bff2d). This user is authorized, but they must confirm their affiliation with their company. Start the authorization process by clicking here, click "Corporate", select the appropriate company from the list, then confirm your affiliation on the page that appears. For further assistance with EasyCLA, please submit a support request ticket.

[benjie]

@xpanvictor Though this is a single character change, we cannot merge it without the CLA being signed. Are you happy to do so?

[bignimbus]

Hi @xpanvictor! Thanks for opening this up. Since this PR has been blocked by the CLA check for a while, we're closing it. But we've addressed the typo on a separate branch. Thanks!