Change use of expr index in Bash string operations lessons

[marktlinn]

First of all great work on putting the bash scripting course together, you're providing valuable content to the community and doing a great service for the coding community. Thanks for that 🙏

In the Bash String Operations lesson (https://www.learnshell.org/en/Basic_String_Operations) the expr index is used to show vairous forms of string manipulation, but from the index operator is not included in all versions of expr, and perhaps a more standardised way of doing these manipulations and operations in Bash would be better for people learning.