clean up whitespace

[henrytill]

Thanks for this amazing resource!

I noticed there is a seemingly inconsistent mix of tabs and spaces across the repo.

This PR converts the tabs to (8) spaces and removes a few instances of trailing whitespace.

This was achieved using the following script:

#!/bin/sh

set -e

list_files() {
    git ls-files
}

remove_trailing_whitespace() {
    while read -r input
    do
        sed -i 's/[ \t]*$//' $input
    done
}

untabify() {
    while read -r input
    do
        expand -t 8 $input | sponge $input
    done
}

list_files | untabify

list_files | remove_trailing_whitespace