Update registerExtension API doc link

[cacheung]

Update registerExtension API doc link

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #93 (33eeec3) into dev-v2.0.0 (7354c19) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           dev-v2.0.0      #93   +/-   ##
===========================================
  Coverage       93.36%   93.36%           
===========================================
  Files              12       12           
  Lines             678      678           
  Branches          103      103           
===========================================
  Hits              633      633           
  Misses             16       16           
  Partials           29       29           

Flag	Coverage Δ
functional-tests	67.55% <ø> (ø)
unit-tests	93.07% <ø> (ø)

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