Fix filename in external-files-processes.adoc

[Codearella]

I don't think these were supposed to have an underscore at the end - it appears to be a typo. (external_fieldname_ should be external_fieldname)

https://issues.apache.org/jira/browse/SOLR-XXXXX

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[HoustonPutman]

Looked at the code and this looks correct!

Thanks for finding and fixing this 🙂