Fix spelling error in README (#3069)

This PR resolves the spelling error in the "Contributing" section of the
README. The sentence "Contribute the language design: feedback on
design, new design proposal" has been corrected to "Contribute to the
language design: feedback on design, new design proposal". This change
enhances the clarity and accuracy of the contribution guidelines.

Closes #3068

Co-authored-by: aroramrinaal <aroramrinaal@gmail.com>

README.md

@@ -337,7 +337,7 @@ contribute.
 
 You can also directly:
 
--   [Contribute the language design](CONTRIBUTING.md#contributing-to-the-language-design):
+-   [Contribute to the language design](CONTRIBUTING.md#contributing-to-the-language-design):
     feedback on design, new design proposal
 -   [Contribute to the language implementation](CONTRIBUTING.md#contributing-to-the-language-implementation)
     -   [Carbon Explorer](/explorer/): bug report, bug fix, language feature