Fix SCANN index type used ivf_flat value

[congqixia]

See also #594
/kind bug

[sre-ci-robot]

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[codecov]

Codecov Report

Merging #597 (dccf3a8) into master (b1c9ccb) will not change coverage.
Report is 3 commits behind head on master.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #597   +/-   ##
=======================================
  Coverage   82.92%   82.92%           
=======================================
  Files          40       40           
  Lines        3643     3643           
=======================================
  Hits         3021     3021           
  Misses        511      511           
  Partials      111      111           

Files	Coverage Δ
entity/indexes_gen.go	99.25% <100.00%> (ø)
entity/indexes_search_param_gen.go	98.34% <ø> (ø)

[yah01]

/lgtm