message: Reword the curl proxy fix changelog entry.
diff: ### Bugs Fixed
  -- [[#3537]](https://github.com/Azure/azure-sdk-for-cpp/issues/3537) Updated field type `CurlTransportOptions.Proxy` from `std::string` to `Azure::Nullable<std::string>`. This change allow to se an empty string to make libcurl ignore proxy settings from environment.
  +- [[#3537]](https://github.com/Azure/azure-sdk-for-cpp/issues/3537) Updated field type `CurlTransportOptions.Proxy` from `std::string` to `Azure::Nullable<std::string>`. This allows libcurl to ignore the proxy settings from the environment when the string is empty.
  - [[#3548]](https://github.com/Azure/azure-sdk-for-cpp/issues/3548), [[#1098]](https://github.com/Azure/azure-sdk-for-cpp/issues/1098) Improve performance of the Http transport on Windows by reusing the same session handle across all requests.
  ### Other Changes
  