Adding -i in shell command for input file Documentation instructions

What does this MR do and why?

the actual command in the documentation

base64 /path/to/sa-key.json | tr -d \\n

fails, and since the step by step leads to download a key file, the command should be:

base64 -i /path/to/sa-key.json | tr -d \\n

using -i to indicate the input file

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