Fix typo in index.html template comment

[acusti]

this is a very straightforward typo fix. the cra-template and cra-template-typescript template/public/index.html file includes an HTML comment giving instructions on running the project and creating a production bundle. currently, the first sentence is:

To begin the development, run npm start or yarn start.

this PR changes the comment in both files to:

To begin the development server, run npm start or yarn start.

[stale]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.

[acusti]

this is a trivial thing to review or merge. it just adds a word to the instructions in an HTML comment to make the sentence clearer and fix its grammar.