Fix the format for adding readers' answers

Author: ryuichiueda

answer/20.md

@@ -6,5 +6,12 @@ $ for i in $(cd /usr;echo *); do echo $i ; done
 ```
 別解1（上田）$ for f in /usr/* ; do echo ${f##*/} ; done
 別解2（上田）$ a=(/usr/*); echo -e ${a[@]/\/usr\//\\n}
+```
+
+### 読者別解
+
+by [kariya-mitsuru](https://github.com/kariya-mitsuru)
+
+```
 別解3 $ (cd /usr; set -- *; IFS=$'\n'; echo "$*")
 ```