Fix link to "gRPC API configuration" in README

[alpire]

https://grpc-ecosystem.github.io/grpc-gateway/docs/grpcapiconfiguration.html leads to a 404.

References to other Issues or PRs

N/A

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Fix link to "gRPC API configuration" in README. The current one leads to a 404.

Other comments

[codecov-io]

Codecov Report

Merging #1873 (c9481a8) into master (61c450d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1873   +/-   ##
=======================================
  Coverage   57.96%   57.96%           
=======================================
  Files          34       34           
  Lines        3771     3771           
=======================================
  Hits         2186     2186           
  Misses       1311     1311           
  Partials      274      274           

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[johanbrandhorst]

Thanks for your contribution!