docs: fix vim.filetype.add by avoiding quotes (#19433)

* Problem

Quotes are special in doxygen, and should be escaped. *Sometimes* they
cause doc generation issues. Like in #17785

* Solution

Replace double quotes with single quotes

runtime/lua/vim/filetype.lua

@@ -2241,30 +2241,30 @@ end
 --- <pre>
 ---  vim.filetype.add({
 ---    extension = {
----      foo = "fooscript",
+---      foo = 'fooscript',
 ---      bar = function(path, bufnr)
 ---        if some_condition() then
----          return "barscript", function(bufnr)
+---          return 'barscript', function(bufnr)
 ---            -- Set a buffer variable
 ---            vim.b[bufnr].barscript_version = 2
 ---          end
 ---        end
----        return "bar"
+---        return 'bar'
 ---      end,
 ---    },
 ---    filename = {
----      [".foorc"] = "toml",
----      ["/etc/foo/config"] = "toml",
+---      ['.foorc'] = 'toml',
+---      ['/etc/foo/config'] = 'toml',
 ---    },
 ---    pattern = {
----      [".*/etc/foo/.*"] = "fooscript",
+---      ['.*/etc/foo/.*'] = 'fooscript',
 ---      -- Using an optional priority
----      [".*/etc/foo/.*%.conf"] = { "dosini", { priority = 10 } },
----      ["README.(%a+)$"] = function(path, bufnr, ext)
----        if ext == "md" then
----          return "markdown"
----        elseif ext == "rst" then
----          return "rst"
+---      ['.*/etc/foo/.*%.conf'] = { 'dosini', { priority = 10 } },
+---      ['README.(%a+)$'] = function(path, bufnr, ext)
+---        if ext == 'md' then
+---          return 'markdown'
+---        elseif ext == 'rst' then
+---          return 'rst'
 ---        end
 ---      end,
 ---    },