Update macOS build instructions

[WillGreen]

Makes three README changes to help Mac users building tic80.

1. DCMAKE_POLICY_VERSION_MINIMUM=3.5 required with brew version of cmake
2. Correct binary name for chmod +x (binary is called tic80, not TIC80dev)
3. Renamed app from TIC80DEV to tic80dev for consistency with release name

I also removed the scale param from the exec script because it caused tic80 to open in a tiny window. The default scaling seems to work better (and matches the release version).

I have tested this with:

• cmake version 4.0.3 - from Brew
• Apple clang version 17.0.0 (clang-1700.0.13.5)
• macOS 15.5
• CPU M1 Max (arm64)

Without the first change, builds with the current brew version of cmake fail with:

CMake Error at vendor/zip/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.