[UX] Document settings language simplification (#3215)

As suggested by @comradekingu in https://github.com/koreader/koreader/pull/3165#issuecomment-326795203

frontend/apps/reader/modules/readerfont.lua

@@ -15,7 +15,7 @@ local ReaderFont = InputContainer:new{
     font_face = nil,
     font_size = nil,
     line_space_percent = nil,
-    font_menu_title = _("Change font"),
+    font_menu_title = _("Font"),
     face_table = nil,
     -- default gamma from crengine's lvfntman.cpp
     gamma_index = nil,

frontend/apps/reader/modules/readerhighlight.lua

@@ -68,7 +68,7 @@ end
 function ReaderHighlight:addToMainMenu(menu_items)
     -- insert table to main reader menu
     menu_items.highlight_options = {
-        text = _("Highlight options"),
+        text = _("Highlighting"),
         sub_item_table = self:genHighlightDrawerMenu(),
     }
 end