fix link error under preview (#3566)

templates/repo/editor/edit.tmpl

@@ -30,7 +30,7 @@
 				<div class="ui top attached tabular menu" data-write="write" data-preview="preview" data-diff="diff">
 					<a class="active item" data-tab="write"><i class="octicon octicon-code"></i> {{if .IsNewFile}}{{.i18n.Tr "repo.editor.new_file"}}{{else}}{{.i18n.Tr "repo.editor.edit_file"}}{{end}}</a>
 					{{if not .IsNewFile}}
-					<a class="item" data-tab="preview" data-url="{{AppSubUrl}}/api/v1/markdown" data-context="{{.RepoLink}}" data-preview-file-modes="{{.PreviewableFileModes}}"><i class="octicon octicon-eye"></i> {{.i18n.Tr "repo.release.preview"}}</a>
+					<a class="item" data-tab="preview" data-url="{{AppSubUrl}}/api/v1/markdown" data-context="{{.RepoLink}}/src/{{.BranchName}}" data-preview-file-modes="{{.PreviewableFileModes}}"><i class="octicon octicon-eye"></i> {{.i18n.Tr "repo.release.preview"}}</a>
 					<a class="item" data-tab="diff" data-url="{{.RepoLink}}/_preview/{{.BranchName}}/{{.TreePath}}" data-context="{{.BranchLink}}"><i class="octicon octicon-diff"></i> {{.i18n.Tr "repo.editor.preview_changes"}}</a>
 					{{end}}
 				</div>