small wording nit

suggestions from @jolheiser - thx

Author: 6543

routers/repo/issue.go

@@ -1449,7 +1449,7 @@ func ChangeIssueReaction(ctx *context.Context, form auth.ReactionForm) {
 	switch ctx.Params(":action") {
 	case "react":
 		if !util.IsStringInSlice(form.Content, setting.UI.Reactions) {
-			log.Error("ChangeIssueReaction: Reaction '%s' is no allowed reaction", form.Content)
+			log.Error("ChangeIssueReaction: '%s' is not an allowed reaction", form.Content)
 			return
 		}
 
@@ -1549,7 +1549,7 @@ func ChangeCommentReaction(ctx *context.Context, form auth.ReactionForm) {
 	switch ctx.Params(":action") {
 	case "react":
 		if !util.IsStringInSlice(form.Content, setting.UI.Reactions) {
-			log.Error("ChangeIssueReaction: Reaction '%s' is no allowed reaction", form.Content)
+			log.Error("ChangeIssueReaction: '%s' is not an allowed reaction", form.Content)
 			return
 		}