Fixed a spelling error in README

[Iron-56]

Changes: Fixed a spelling error in README.md of p5 web editor.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines

@davepagurek @ksen0 @raclim Please review the PR

[raclim]

Thank you so much @Iron-56 for catching this and quickly following up with a solution! I added in a quick comment, but overall I think it's good to go—I'll merge this in to have something is place!