Update how-to-write-a-transformer.md

[r0zar]

Update bad link for plugin authoring.

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #4902 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #4902    +/-   ##
========================================
  Coverage   60.50%   60.50%            
========================================
  Files         350      350            
  Lines       15208    15208            
  Branches     2882     3048   +166     
========================================
  Hits         9201     9201            
+ Misses       5519     5501    -18     
- Partials      488      506    +18     

Impacted Files	Coverage Δ
packages/amplify-util-mock/src/api/api.ts	0.00% <0.00%> (ø)
packages/graphql-mapping-template/src/print.ts	35.71% <0.00%> (ø)
packages/amplify-util-mock/src/storage/storage.ts	0.00% <0.00%> (ø)
...ges/amplify-util-mock/src/CFNParser/stack/index.ts	88.09% <0.00%> (ø)
...es/amplify-util-mock/src/api/resolver-overrides.ts	0.00% <0.00%> (ø)
...es/graphql-transformer-core/src/stripDirectives.ts	35.29% <0.00%> (ø)
.../amplify-util-mock/src/utils/lambda/loadMinimal.ts	0.00% <0.00%> (ø)
...graphql-transformer-core/src/TransformerContext.ts	26.10% <0.00%> (ø)
...mplify-appsync-simulator/src/resolvers/function.ts	15.38% <0.00%> (ø)
...plify-appsync-simulator/src/server/subscription.ts	30.43% <0.00%> (ø)
... and 7 more

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[jhockett]

LGTM!

[github-actions]

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