fix bleurt docs

[bricksdont]

Fixes several confusing things about the BLEURT metric

• A logging warning states that "BLEURT-Base" is used as the model checkpoint, making it seem like "BLEURT-Base" is a model checkpoint identifier (it's not). 'bleurt-base-128' is the actual default
• The README mentions a checkpoint argument, but the correct argument is config_name. Also, the way that the documentation is arranged it seems that checkpoint is an argument of compute, but it belongs to load

Fixes / combines #617 and #679

[lhoestq]

lgtm