possessive "it" is "its"

…can be used

This may help hammer home the point for new users who don't know the
clap API and might not realize this is a lot of functionality.

When viewing examples, it is very hard to figure out what they do without running them, which requires substantial efforts.  I have added the output of all examples at the top.

Also, two tiny spelling mistakes - in the readme and in the rename_all.

In case anyone wants to auto-generate something similar, I used this helper script, and made a few minor line manipulations afterwards.

```bash

run() {
  RES=$(cargo run -q --example "$1" -- --help 2> /dev/null | sed -e 's#^#//! #' )
  { echo "//! Running this example with --help prints this message:" ; echo "//! -----------------------------------------------------" ; echo "$RES" ; echo "//! -----------------------------------------------------" ; echo "" ; cat "examples/$1.rs" ; } > "$1.rs"
  rm "examples/$1.rs"
  mv "$1.rs" "examples/"
}

run after_help
run at_least_two
```

fixes #521