Adds 2019, Day 18 to main ToC

Author: romellem

README.md

@@ -49,7 +49,7 @@ Completed: **25 / 25** (100% ⭐️)
 
 ## 2019 Puzzles List
 
-Completed: **17 / 25** (68%)
+Completed: **18 / 25** (72%)
 
 *  [✅ Day 1](2019/1/)
 *  [✅ Day 2](2019/2/)
@@ -68,7 +68,7 @@ Completed: **17 / 25** (68%)
 *  Day 15
 *  [✅ Day 16](2019/16/)
 *  Day 17
-*  Day 18
+*  [✅ Day 18](2019/18/)
 *  [✅ Day 19](2019/19/)
 *  Day 20
 *  Day 21