macOS installer: Properly pluralise "architectures" - #160759

The macOS installer says:

This package will install Python 3.15.2 for macOS 10.15 or later for the following architecture(s): arm64, x86_64.

Let's fix that to say:

This package will install Python 3.15.2 for macOS 10.15 or later for the following architectures: arm64, x86_64.

And soon:

This package will install Python 3.15.2 for macOS 10.15 or later for the following architecture: arm64.