doc update docker tag command to use existing variable

[pavel-balas]

Issue #, if available:

The command in the documentation refers to the SPARK_REPOSITORY_NAME environment variable which doesn't exists. The documentation mention only SPARK_REPOSITORY variable.

Description of changes:

Use SPARK_REPOSITORY in the docker tag command tag.

I'm also doing two more small changes

• adding exact command aws ecr create-repository where only link to the aws official doc is mentioned.
• wrapping environment variables into brackets (e.g. ${VARIABLE}). The colon (:) character may mean formatting for variable so wrapping brackets are safer

% echo $SPARK_REPOSITORY:latest  
sagemaker-sparkatest

% echo ${SPARK_REPOSITORY}:latest
sagemaker-spark:latest

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