Meta: Promote RISC-V as supported in the README

RISC-V now works just as well as Arm and even passes more tests than it.

"riscv64" is not an official name for the RISC-V architecture and
just using the pretty-printed name for RISC-V looks weird, so use the
pretty name for all architectures instead.

README.md

@@ -1,6 +1,6 @@
 # SerenityOS
 
-Graphical Unix-like operating system for x86-64 and aarch64 computers.
+Graphical Unix-like operating system for 64-bit x86, Arm, and RISC-V computers.
 
 [![GitHub Actions Status](https://github.com/SerenityOS/serenity/workflows/Build,%20lint,%20and%20test/badge.svg)](https://github.com/SerenityOS/serenity/actions?query=workflow%3A"Build%2C%20lint%2C%20and%20test")
 [![Fuzzing Status](https://oss-fuzz-build-logs.storage.googleapis.com/badges/serenity.svg)](https://bugs.chromium.org/p/oss-fuzz/issues/list?sort=-opened&can=1&q=proj:serenity)