docs: fix Cat constructor in README example

[thehanslevi]

Summary

• Add the missing Cat(const std::string& name) declaration to the README quick-start example.
• Keep the declaration aligned with luaCat.ctor<std::string>(), AwesomeCat.new("Bingo"), and the full example/example.cpp implementation.

Fixes #44.

Validation

• rg -n "Cat\\(|ctor<std::string>|AwesomeCat\\.new|Cat\\(const std::string" README.md example/example.cpp example/example.lua
• Compiler check: the current README-style declaration with only Cat() rejects Cat(std::string("Bingo")).
• Compiler check: adding Cat(const std::string& name) accepts Cat(std::string("Bingo")).
• git -c core.whitespace=cr-at-eol diff --check

I did not run the full Lua example build because Lua headers/libraries are not installed in my local environment.