Update solution.md

Author: whatsyourask

pwnable.kr/flag/solution.md

@@ -1,3 +1,3 @@
 Firstly, `strings -n 8 flag` gives a result with 'UPX service'.
-Secondly, `upx -d flag' and we now got the normal executable file.
+Secondly, `upx -d flag` and we now got the normal executable file.
 Thirdly, some sort of debugging: set a breakpoint in the end of the main function, then try to read the address of flag and you'll get it.