Fix svgo example in readme

[maple3142]

The usage are changed after #354: imagemin/imagemin-svgo@43888a0

[bdukes]

Note that the configuration documentation states:

The default preset of plugins is fully overridden if the plugins field is specified. Use preset-default plugin to customize plugins options.

So the default example may want to include that (or at least a note) so that folks don't think they're only altering one plugin when they're actually removing all of the default plugins.

Maybe this example:

svgo({
  plugins: [
    {
      name: "preset-default",
      params: { overrides: { removeViewBox: false, removeUselessDefs: false } },
    },
  ],
})

[bdukes]

So that other can find this when searching GitHub, the error you see when you're using the older SVGO config is

Plugin name should be specified