Fixed simple grammatical mistake

[andreyseas]

Why are these changes needed?

Was asked in the Discord office hour to review and submit PR for the new Tutorial section if there are any spelling or grammatical mistakes. This update, although small, assists with readability. :)

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.