Update apollo-link-context.md

[glenndixon]

Correct language with regard to behavior of setContext link. The returned object doesn't become the new context, it's merged with the current context:

https://github.com/apollographql/apollo-client/blob/main/src/link/utils/createOperation.ts#L8

Checklist:

• If this PR contains changes to the library itself (not necessary for e.g. docs updates), please include a changeset (see CONTRIBUTING.md)
• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests