rename some variables in scrapely.regionextract

[kmike]

I've tried to make code a bit more readable by renaming some variables:

• https://github.com/scrapy/scrapely/blob/664453340e072d2d9db967bc38ff17b2d7b4ef42/scrapely/extraction/regionextract.py#L365 'region_elements' argument doesn't contain PageRegion objects - it usually contains RecordExtractor, RepeatedDataExtractor, AdjacentVariantExtractor, etc., so it is renamed to 'extractors';
• https://github.com/scrapy/scrapely/blob/664453340e072d2d9db967bc38ff17b2d7b4ef42/scrapely/extraction/regionextract.py#L391 - variable is named 'end_region', but it contains neither page region nor an extractor object - it is an integer index, so it is renamed to 'end_index_exclusive'.