Should fix a comment referring to stats collector?

Hi,

Statistics collector has been removed since 5891c7a8ed8f2d3d5, but there 
was a comment referring 'statistics collector' in pg_statistic.h.

>  Note that since the arrays are variable-size, K may be chosen by the 
> statistics collector.

Should it be modified to 'cumulative statistics system' like manual on 
monitoring stats[1]?
Its title has changed from 'statistics collector' to 'cumulative 
statistics system'.

[1] https://www.postgresql.org/docs/current/monitoring-stats.html

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Regards,

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Atsushi Torikoshi
NTT DATA CORPORATION