Ensure symbol links end with a slash

This removes the need for extra redirects when hosting the data on
Cloudflare R2.

Changelog: fixed

src/idoc/module_name.inko

@@ -46,7 +46,7 @@ class ModuleName {
   }
 
   fn symbol_link(name: String) -> String {
-    '${link}${name}'
+    '${link}${name}/'
   }
 
   fn link -> String {

test/idoc/test_module_name.inko

@@ -69,7 +69,7 @@ fn pub tests(t: mut Tests) {
   t.test('ModuleName.symbol_link', fn (t) {
     let name = ModuleName.new('std.foo')
 
-    t.equal(name.symbol_link('A'), '/module/std/foo/A')
+    t.equal(name.symbol_link('A'), '/module/std/foo/A/')
   })
 
   t.test('ModuleName.link', fn (t) {

test/idoc/test_url.inko

@@ -12,5 +12,6 @@ fn pub tests(t: mut Tests) {
     t.equal(relative_url(from: '/foo/bar', to: 'test.md'), '../test.md')
     t.equal(relative_url(from: '/foo/bar', to: '../test.md'), '../../test.md')
     t.equal(relative_url(from: '/foo/bar', to: 'foo'), '../foo')
+    t.equal(relative_url(from: '/foo/bar', to: 'foo/'), '../foo/')
   })
 }