Fix typo

Change-Id: I2bb8daba6878432c002471bdfa33652147c3b6f9
Reviewed-on: https://gerrit.libreoffice.org/65602
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

bin/unpack-sources

@@ -10,7 +10,7 @@
 
 usage()
 {
-    echo "Helper script to unpack the LO source tarbals"
+    echo "Helper script to unpack the LO source tarballs"
     echo
     echo "Usage: ${0##*/} [--help] start-dir tarball..."
     echo