[2023-10] Firm up the wording for "executing selection set serially" example

[benjie]

Lee commented that the example @rivantsov pointed out in graphql/graphql-spec#984 is indeed confusing, but that just adding mutation would be an incomplete fix to the issue due to the leading text. Benjie to improve the leading wording.

Related:

• added mutation op type to examples 193, 194 graphql-spec#984

• [2022-12] Clarify the selection set in the example is a selection set and not a document #1335

• assignee(s): @benjie

• source: https://github.com/graphql/graphql-wg/blob/main/notes/2023/2023-10.md#review-previous-meetings-action-items-5m-lee

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[benjie]

This has been addressed in graphql/graphql-spec#1032