Fix ambiguous Get() examples in README after new Params overload

[Copilot]

The new Get(path, const Params&, ...) overload added ambiguity for existing call sites using bare brace-enclosed initializer lists, since Headers and Params are both multimap<std::string, std::string> typedefs. The README's own documented examples for Get() with headers no longer compile as a result.

Changes

• Wrapped bare {{...}} initializer lists in explicit httplib::Headers{...} for all affected Get() examples in README.md:
  • "GET with HTTP headers" example
  • "Range" example (with make_range_header)
  • "Default Accept-Encoding value" examples (two call sites)

Example

// Before: ambiguous between Get(path, Params, progress) and Get(path, Headers, progress)
auto res = cli.Get("/hi", {{"Hello", "World!"}});

// After: explicit type disambiguates the call
auto res = cli.Get("/hi", httplib::Headers{{"Hello", "World!"}});

All updated snippets were compiled directly against httplib.h to confirm they no longer hit the "ambiguous call to overloaded function" error.