NIFI-8952: Fixed logback.xml files BOOTSTRAP_FILE appender commentary

[kevinbsilva]

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Description of PR

Correction of loback.xml files commentary for BOOTSTRAP_FILE appender. Created Jira Issue NIFI-8952

The logback.xml commentary for BOOTSTRAP_FILE refers to the app log, which can cause misinterpretation when
updating the logback configuration.

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[exceptionfactory]

Thanks for correcting the comments @kevinbsilva! Looks good, +1 merging.