Grammar update on Minimize main thread work

[Codimart]

grammar check and improved it

Fixes #SOME_ISSUE_NUMBER

Changes proposed in this pull request:

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[chrome-devrel-infra[bot]]

:white_check_mark: Preview (static) for commit 5cdc06477de88f7f93308a59210a627194f014f7 available at https://pr-7375-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-7375-static-dot-dcc-staging.uc.r.appspot.com/docs/lighthouse/performance/mainthread-work-breakdown/

[alexandrascript]

@Codimart please sign the CLA so we can merge your corrections :) https://cla.developers.google.com/