Fix path to proc-macro2 in rules.mk

Bug: None
Test: build.py
Change-Id: I4fbaece26108765f10278f6dc70e2f92aee7bcf3
diff --git a/rules.mk b/rules.mk
index ec270e1..0957253 100644
--- a/rules.mk
+++ b/rules.mk
@@ -10,7 +10,7 @@
 MODULE_SRCS := $(LOCAL_DIR)/src/lib.rs
 MODULE_RUST_EDITION := 2018
 MODULE_LIBRARY_DEPS := \
-	external/rust/crates/proc_macro2 \
+	external/rust/crates/proc-macro2 \
 	external/rust/crates/quote \
 	external/rust/crates/syn