Makefile: Correctly document the Apollo3 support

Signed-off-by: Alistair Francis <alistair@alistair23.me>

Makefile

diff --git a/Makefile b/Makefile
index f1f6dd5..b7eef32 100644
--- a/Makefile
+++ b/Makefile
@@ -14,6 +14,7 @@
 	@echo " - opentitan"
 	@echo " - hifive1"
 	@echo " - nrf52"
+	@echo " - apollo3"
 	@echo
 	@echo "Run 'make setup' to setup Rust to build libtock-rs."
 	@echo "Run 'make <board>' to build libtock-rs for that board"