Fix Makefile variable (#282)

Need `$$` for shell variable references. The outer `$` is consumed by make

Makefile

@@ -23,7 +23,7 @@ check : build doclint lint spellcheck check-fmt test
 test :
 	$(CARGO) test --all-features
 	# And run the example tests separately to avoid feature combinations
-	cd examples; for dir in *; do cargo +stable test -p $dir --all-features; done
+	cd examples; for dir in *; do cargo +stable test -p $$dir --all-features; done
 
 clean :
 	$(CARGO) clean