Correct Typo to "API Server"

[ronaldngounou]

Describe the problem
The yellow box should be "API Server" instead of "API Service."

References
Please include a link to the lines where the error appears.

https://github.com/aws/aws-eks-best-practices/blob/4ba97d8a08590c8700dc395639863ea4b8332279/latest/bpg/images/scalability/keeping-up.png