change some grammar mistakes

[XuanhaoZhao]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [x] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfils these requirements:

• [x] It's submitted to the main branch for v2.x (or to a previous version branch)
• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: I'm new to open source. I like vue so much, and it is an awesome tool. But I find two grammar mistakes in the first two paragraphs in README.md. I think you can consider fixing these two mistakes since open source is a noun and "open-source" is an adjective and the phrase is "consider to do/doing sth". Sorry for wasting your time.