[Qt] fix missing plural form for a string in sendcoinsdialog

- this was reported on Transifex by pryds (Thank you!)
[Qt] change some strings to reflect name change to Bitcoin Core
[Qt] fix a translation that occurs 2 times but was a little different
Merge pull request #5341

4709160 [Qt] fix a translation that occurs 2 times but was a little different (Philip Kaufmann)
50db7d9 [Qt] change some strings to reflect name change to Bitcoin Core (Philip Kaufmann)
2747f7c [Qt] fix missing plural form for a string in sendcoinsdialog (Philip Kaufmann)