Documentation: fix broken `sampleRate` in gitpacking(7)

Documentation: fix broken `sampleRate` in gitpacking(7)

The documentation explaining some sample configurations for bitmap
pseudo-merges incorrectly uses a sample rate outside of the allowed
(0,1] range.

This dates back to faf558b23ef (pseudo-merge: implement support for
selecting pseudo-merge commits, 2024-05-23), and was likely written when
the allowable range for this configuration was the integral values
between (0,100].

Fix this to conform to the actual allowable range for this
configuration.

Noticed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>