Fix failing MVSC benchmark builds (#1573)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1573

Noticed a recent stack of commits cause the MVSC builds of benchmark to fail. This was due to forgetting to call `.string()` of a path and trying to escape a character that cannot be escaped.

Reviewed By: philIip

Differential Revision: D53461723

fbshipit-source-id: b6cc034d53b3a61929012965e257a3984c3bff47

capture/NodeToString.cpp

@@ -33,7 +33,7 @@ static void appendYGValueIfNotDefault(
     } else if (value.unit == YGUnitUndefined) {
       j["style"][key] = "undefined";
     } else {
-      std::string unit = value.unit == YGUnitPoint ? "px" : "%%";
+      std::string unit = value.unit == YGUnitPoint ? "px" : "pct";
       j["style"][key]["value"] = value.value;
       j["style"][key]["unit"] = unit;
     }