9.4.15-0+deb8u1) is not available, but a newer one is (9.4.18-0+deb8u1). We redirected you there.
/usr/share/postgresql/9.4/tsearch_data/xsyn_sample.rules is in postgresql-9.4 9.4.18-0+deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | # Sample rules file for eXtended Synonym (xsyn) dictionary
# format is as follows:
#
# word synonym1 synonym2 ...
#
supernova sn sne 1987a
|