fix(man.lua): set modifiable before writing page

(cherry picked from commit 4282c8b41d)

runtime/lua/man.lua

@@ -447,7 +447,7 @@ local function get_page(path, silent)
 end
 
 local function put_page(page)
-  vim.bo.modified = true
+  vim.bo.modifiable = true
   vim.bo.readonly = false
   vim.bo.swapfile = false