Promtail documentation: fix template example for regexReplaceAll function

[sergeykranga]

Fixes an example for regexReplaceAll template function:

• use correct function name
• add missing $ sign when referencing a regex submatch

What this PR does / why we need it:
Updates typos in promtail configuration example

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

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