Change indefinite article "an" to "a" before
the word unique and some variants.

Change-Id: Ia14a6f3b9ec6b257ad8bed3b089faa9b6f07d44f
Reviewed-on: https://gerrit.libreoffice.org/70174
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>