ReadMe.rst: Fix header formatting, path separators, and grammar

[pohaosu]

• Correct header underline lengths to meet ReStructuredText standards.
• Standardize path separators to forward slashes for consistency.
• Fix grammar and improve phrasing in the Submodules section.

Description

• Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
• Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

Integration Instructions

[tianocore-pr-automation]

Pull Request Formatting Issues

⚠️ Remove the following template lines from your PR description:
<_Include a description of the change and why this change was made._>
<_For each item, place an "x" in between [and]if true. Example:[x] (you can also check items in GitHub UI)_>
<_Create the PR as a Draft PR if it is only created to run CI checks._>
<_Delete lines in \<\> tags before creating the PR._>
<_Describe the test(s) that were run to verify the changes._>
<_Describe how these changes should be integrated. Use N/A if nothing is required._>

Address these issues and the validation will automatically re-run when you update your pull request.

[ardbiesheuvel]

Please don't merge back the master branch into your topic branch - this will be taken care of at merge time.

[pohaosu]

@ardbiesheuvel oh, thank you for the reminding! Just reverted them