json-for-modern-cpp: use $() to reference make variables instead of ${}

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/json-for-modern-cpp/json-for-modern-cpp.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 JSON_FOR_MODERN_CPP_VERSION = v3.1.2
-JSON_FOR_MODERN_CPP_SOURCE = json-${JSON_FOR_MODERN_CPP_VERSION}.tar.gz
+JSON_FOR_MODERN_CPP_SOURCE = json-$(JSON_FOR_MODERN_CPP_VERSION).tar.gz
 JSON_FOR_MODERN_CPP_SITE = $(call github,nlohmann,json,$(JSON_FOR_MODERN_CPP_VERSION))
 JSON_FOR_MODERN_CPP_LICENSE = MIT
 JSON_FOR_MODERN_CPP_LICENSE_FILES = LICENSE.MIT