Fix table rendering in web

Gogs does not render simple newline.

Guides/vim.md

@@ -56,11 +56,13 @@ __vimdiff__ basics
 
 Three files : LOCAL | BASE | REMOTE
 
+```txt
 +--------------------------------+
 | LOCAL  |     BASE     | REMOTE |
 +--------------------------------+
 |             MERGED             |
 +--------------------------------+
+```
 
 Three commands : `:diffget LO`, `:diffget BO`, `:diffget RE`