Update README.md: typo fixed, version 3.X vs 2.X

[wstein]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Swagger Codegen 3.X content should start with:

Swagger Codegen 3.X supports OpenAPI version 3
not:
Swagger Codegen 2.X supports OpenAPI version 3

[HugoMario]

please build

[HugoMario]

thanks @wstein !