typo fix

lib/rules_inline/strikethrough.js

@@ -23,7 +23,7 @@ module.exports = function strikethrough(state) {
   // this code also prevents recursion
   if (state.validateInsideEm || state.validateInsideLink) { return false; }
 
-  if (state.level >= state.options.level) { return false; }
+  if (state.level >= state.options.maxNesting) { return false; }
 
   lastChar = start > 0 ? state.src.charCodeAt(start - 1) : -1;
   nextChar = state.src.charCodeAt(start + 2);