Use correct README link to render the README (#23152)

`renderReadmeFile` needs `readmeTreelink` as parameter but gets
`treeLink`.
The values of them look like as following:
`treeLink`:  `/{OwnerName}/{RepoName}/src/branch/{BranchName}`
`readmeTreelink`:
`/{OwnerName}/{RepoName}/src/branch/{BranchName}/{ReadmeFileName}`

`path.Dir` in

8540fc45b1/routers/web/repo/view.go (L316)
should convert `readmeTreelink` into
`/{OwnerName}/{RepoName}/src/branch/{BranchName}` instead of the current
`/{OwnerName}/{RepoName}/src/branch`.

Fixes #23151

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/view.go

@@ -186,7 +186,7 @@ func renderDirectory(ctx *context.Context, treeLink string) {
 		return
 	}
 
-	renderReadmeFile(ctx, readmeFile, treeLink)
+	renderReadmeFile(ctx, readmeFile, fmt.Sprintf("%s/%s", treeLink, readmeFile.name))
 }
 
 // localizedExtensions prepends the provided language code with and without a