optimize indexTypes and metricType for easy using

[nameczz]

Using IndexType and MetricType in our doc example, but it can't working because of wrong type. Fixed it for easy using.

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This pull-request has been approved by: nameczz, shanghaikid

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[codecov]

Codecov Report

Merging #138 (601876a) into main (64a941b) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 601876a differs from pull request most recent head 3563e90. Consider uploading reports for the commit 3563e90 to get more accurate results

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   99.39%   99.38%   -0.01%     
==========================================
  Files          22       22              
  Lines         985      983       -2     
  Branches      193      193              
==========================================
- Hits          979      977       -2     
  Misses          6        6              

Impacted Files	Coverage Δ
milvus/const/Milvus.ts	100.00% <100.00%> (ø)

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