Fix broken link to href-no-hash eslint rule (facebook#3460)

This fixes the broken link to removed `href-no-hash.md` file, that is no longer available in the [eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y) plugin (since [6.0.0](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/a14617528cda23c6c88902ace1d083e4e13ac271/CHANGELOG.md))

packages/eslint-config-react-app/README.md

@@ -45,7 +45,7 @@ The following rules from the [eslint-plugin-jsx-a11y](https://github.com/evcohen
 - [aria-role](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/aria-role.md)
 - [aria-unsupported-elements](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/aria-unsupported-elements.md)
 - [heading-has-content](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/heading-has-content.md)
-- [href-no-hash](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/href-no-hash.md)
+- [href-no-hash](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/v5.1.1/docs/rules/href-no-hash.md)
 - [iframe-has-title](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/iframe-has-title.md)
 - [img-redundant-alt](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/img-redundant-alt.md)
 - [no-access-key](https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/no-access-key.md)