correct giftCertificate i18n key

[mohitb35]

Fixes https://www.notion.so/plantfortheplanet/Fix-Payment-History-Gift-Certificate-Download-Text-translations-e04b17ed804248e7944636b98b37c429

Changes in this pull request:

• corrected the i18n key for giftCertificate so that we see "Gift Certificate" in the download link text seen in Payment History (as opposed to "giftCertificate")

Note:

• this will be seen only if the donation was a gift, and not otherwise (no change to code logic for this).
• Gift Certificate link will be visible only once this issue is resolved - Payment History Download Gift Certificate  #1448

@ankitecd Could you review and merge if OK?

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[norbertschuler]

@mohitb35 please use feature instead of fix for our branches.