[Ingest pipelines] Change copy text from copy to field in Set process…

…or (elastic#205471)

Part of elastic#193186

## Summary
I got some copy change request from docs team after submitting this
[PR](elastic#204336) and I forgot to add
those changes. Adding this PR as a follow-up.

x-pack/platform/plugins/shared/ingest_pipelines/public/application/components/pipeline_editor/components/processor_form/processors/set.tsx

@@ -176,7 +176,7 @@ const fieldsConfig: FieldsConfig = {
     helpText: (
       <FormattedMessage
         id="xpack.ingestPipelines.pipelineEditor.setForm.enablingCopydHelpText"
-        defaultMessage="Define fields to copy into {field} insted of defining {value}."
+        defaultMessage="Specify fields to copy into {field} instead of setting a {value}."
         values={{
           field: <EuiCode>{'Field'}</EuiCode>,
           value: <EuiCode>{'Value'}</EuiCode>,