runtime(menu): define shortcut for File->Open Tab (#12895)

Seems missing as noted by Antonio Giovanni Colombo. So add it and use
the 'T' as shortcut, which does not seem to be used in the File dialog.

Verified on Windows.

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Antonio Giovanni Colombo <azc100@gmail.com>
diff --git a/runtime/menu.vim b/runtime/menu.vim
index 6d12bcd..59879d1 100644
--- a/runtime/menu.vim
+++ b/runtime/menu.vim
@@ -122,7 +122,7 @@
 " File menu
 an 10.310 &File.&Open\.\.\.<Tab>:e		:browse confirm e<CR>
 an 10.320 &File.Sp&lit-Open\.\.\.<Tab>:sp	:browse sp<CR>
-an 10.320 &File.Open\ Tab\.\.\.<Tab>:tabnew	:browse tabnew<CR>
+an 10.320 &File.Open\ &Tab\.\.\.<Tab>:tabnew	:browse tabnew<CR>
 an 10.325 &File.&New<Tab>:enew			:confirm enew<CR>
 an <silent> 10.330 &File.&Close<Tab>:close
 	\ :if winheight(2) < 0 && tabpagewinnr(2) == 0 <Bar>