docs: fix node quickly install with nvm

[Songkunda]

update docs/api/index.md:8 nvm install 7 to nvm install 12

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (632a803) 98.66% compared to head (7f3d3f7) 98.66%.

❗ Current head 7f3d3f7 differs from pull request most recent head 83adab6. Consider uploading reports for the commit 83adab6 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1760   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files           5        5           
  Lines         523      523           
  Branches      146      146           
=======================================
  Hits          516      516           
  Misses          7        7           

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[zacanger]

I know this is already closed, but rather than having a hardcoded version, why not use a flag like --lts?