Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/shell/solutions/hello_world.md

@@ -0,0 +1,15 @@
+## Shell Scripting - Hello World
+
+### Objectives
+
+1. Define a variable with the string 'Hello World'
+2. Print the value of the variable you've defined and redirect the output to the file "amazing_output.txt"
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+HW_STR="Hello World"
+echo $HW_STR > amazing_output.txt
+```