Grammar improvements around backwards-compatibility policy.

[PaarthShah]

What do these changes do?

Improve grammar and brevity in communication in the Policy for Backward Incompatible Changes section of docs/index.rst

Are there changes in behavior for the user?

No functional changes. Will improve introductory communication on https://docs.aiohttp.org

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder

[codecov]

Codecov Report

Merging #6979 (17b8530) into master (eb6b863) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6979   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files         103      103           
  Lines       29957    29957           
  Branches     3633     3633           
=======================================
  Hits        29211    29211           
  Misses        567      567           
  Partials      179      179           

Flag	Coverage Δ
CI-GHA	97.41% <ø> (ø)
OS-Linux	97.04% <ø> (-0.02%)	⬇️
OS-Windows	95.39% <ø> (ø)
OS-macOS	96.68% <ø> (-0.01%)	⬇️
Py-3.10.7	97.21% <ø> (-0.01%)	⬇️
Py-3.11.0-rc.2	96.56% <ø> (ø)
Py-3.7.14	96.88% <ø> (ø)
Py-3.7.9	95.28% <ø> (ø)
Py-3.8.10	95.17% <ø> (-0.01%)	⬇️
Py-3.8.14	96.78% <ø> (ø)
Py-3.9.13	95.17% <ø> (ø)
Py-3.9.14	96.79% <ø> (-0.01%)	⬇️
Py-pypy7.3.9	96.43% <ø> (-0.02%)	⬇️
VM-macos	96.68% <ø> (-0.01%)	⬇️
VM-ubuntu	97.04% <ø> (-0.02%)	⬇️
VM-windows	95.39% <ø> (ø)

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