docs: replace <pre> with ``` (#25136)

runtime/lua/vim/F.lua

@@ -5,13 +5,14 @@ local F = {}
 --- If all arguments are nil, returns nil.
 ---
 --- Examples:
---- <pre>
+---
+--- ```lua
 --- local a = nil
 --- local b = nil
 --- local c = 42
 --- local d = true
 --- assert(vim.F.if_nil(a, b, c, d) == 42)
---- </pre>
+--- ```
 ---
 ---@param ... any
 ---@return any