Fix sed on MacOS

Author: 田凯夫

os/Makefile

@@ -22,7 +22,7 @@ KERNEL_ENTRY_PA := 0x80200000
 OBJDUMP := rust-objdump --arch-name=riscv64
 OBJCOPY := rust-objcopy --binary-architecture=riscv64
 
-CHAPTER ?= $(shell git rev-parse --abbrev-ref HEAD | grep -oP 'ch\K[0-9]')
+CHAPTER ?= $(shell git rev-parse --abbrev-ref HEAD | sed -E 's/ch([0-9])/\1/')
 TEST ?= $(CHAPTER)
 BASE ?= 1