Replace {exercise} stub in READMEs across exercises

[sshine]

Currently, all README.md files have something like:

## Loading your exercise implementation in PolyML

```
$ poly --use {exercise}.sml
```

Or:

```
$ poly
> use "{exercise}.sml";
```

**Note:** You have to replace {exercise}.

Replace {exercise} with the actual exercise name and remove the Note for all exercises.

[sshine]

This was fixed in #115, but later addressed as an issue and reverted in #117.

Closing this issue as the recommendation no longer applies.