Minor: correct a typo.

src/introduction/README.md

@@ -19,7 +19,7 @@ These exercises are designed to be run on the CheriBSD operating system in its
 pure-capability CheriABI process environment.
 They can be run on various instantiations of CHERI-RISC-V, including on QEMU
 and on FPGA implementations.
-QEMU-CHERI is a convenient instruction-set-level emulator, and is usaully the
+QEMU-CHERI is a convenient instruction-set-level emulator, and is usually the
 best starting point for most users (even those intending to eventually run on
 hardware).
 You can use our [cheribuild](https://github.com/CTSRD-CHERI/cheribuild) tool