Fix misleading sentence

[6543]

The counter show's how many unique users who have created issues are now closed.
But you get a other meaning (see screenshot)

ref for .Activity.ClosedIssueAuthorCount:

gitea/models/repo_activity.go

Lines 275 to 280 in 4e7e3bd

	// Closed issue authors
	sess = issuesForActivityStatement(repoID, fromTime, true, false)
	if _, err = sess.Select("count(distinct issue.poster_id) as `count`").Table("issue").Get(&count); err != nil {
	return err
	}
	stats.ClosedIssueAuthorCount = count