Fix two more instances of + signs added to links.

The first is image URLs rendered by markdown.go,
the second is shortlinks rendered by html.go.

However, with this commit I feel more certain
that + was chosen for a reason, so it might be
a good idea to track that down before this PR
is accepted.

modules/markup/html.go

@@ -377,7 +377,7 @@ func RenderShortLinks(rawBytes []byte, urlPrefix string, noLink bool, isWikiMark
 		}
 		absoluteLink := isLink([]byte(link))
 		if !absoluteLink {
-			link = strings.Replace(link, " ", "+", -1)
+			link = strings.Replace(link, " ", "-", -1)
 		}
 		if image {
 			if !absoluteLink {

modules/markup/markdown/markdown.go

@@ -112,7 +112,7 @@ func (r *Renderer) Image(out *bytes.Buffer, link []byte, title []byte, alt []byt
 		} else {
 			lnk := string(link)
 			lnk = util.URLJoin(prefix, lnk)
-			lnk = strings.Replace(lnk, " ", "+", -1)
+			lnk = strings.Replace(lnk, " ", "-", -1)
 			link = []byte(lnk)
 		}
 	}