Change letter being replaced from s to i (#13)

Having people write `s/s/S/` is a little more confusing than `s/i/I/`
for their first exercise.

Author: illicitonion

individual-shell-tools/sed/script-01.sh

@@ -2,6 +2,6 @@
 
 set -euo pipefail
 
-# TODO: Write a command to output input.txt with all occurrences of the letter `s` replaced with `S`.
+# TODO: Write a command to output input.txt with all occurrences of the letter `i` replaced with `I`.
 # The output should contain 11 lines.
-# The first line of the output should be: "ThiS iS a Sample file for experimenting with Sed.".
+# The first line of the output should be: "ThIs Is a sample fIle for experImentIng with sed.".