Fix improper format specifier in printf

%d in format string requires 'int' but the argument type is 'unsigned int'.
diff --git a/src/jsontestrunner/main.cpp b/src/jsontestrunner/main.cpp
index 7105b39..54ca302 100644
--- a/src/jsontestrunner/main.cpp
+++ b/src/jsontestrunner/main.cpp
@@ -106,9 +106,9 @@
     for (Json::ArrayIndex index = 0; index < size; ++index) {
       static char buffer[16];
 #if defined(_MSC_VER) && defined(__STDC_SECURE_LIB__)
-      sprintf_s(buffer, sizeof(buffer), "[%d]", index);
+      sprintf_s(buffer, sizeof(buffer), "[%u]", index);
 #else
-      snprintf(buffer, sizeof(buffer), "[%d]", index);
+      snprintf(buffer, sizeof(buffer), "[%u]", index);
 #endif
       printValueTree(fout, value[index], path + buffer);
     }