Docs show a color param, when the correct param is background

https://github.com/mattbearman/monogram/blob/main/README.md?plain=1#L123

shows:

```ruby
Monogram.new(name: 'custom colour', color: 'rgba(127, 0, 0, 0.8)', shape: Monogram::Avatar::ROUNDED).to_s
```

when it should be:
```ruby
Monogram.new(name: 'custom colour', background: 'rgba(127, 0, 0, 0.8)', shape: Monogram::Avatar::ROUNDED).to_s
```