Fix Readme render bug (#19992)

Fix #19988 

Co-authored-by: zeripath <art27@cantab.net>

routers/web/repo/view.go

@@ -357,7 +357,7 @@ func renderReadmeFile(ctx *context.Context, readmeFile *namedBlob, readmeTreelin
 		var result strings.Builder
 		err := markup.Render(&markup.RenderContext{
 			Ctx:          ctx,
-			RelativePath: ctx.Repo.TreePath,
+			RelativePath: path.Join(ctx.Repo.TreePath, readmeFile.name), // ctx.Repo.TreePath is the directory not the Readme so we must append the Readme filename (and path).
 			URLPrefix:    readmeTreelink,
 			Metas:        ctx.Repo.Repository.ComposeDocumentMetas(),
 			GitRepo:      ctx.Repo.GitRepo,