New memory icon for footer (#5300)

See: https://github.com/koreader/koreader/pull/5203#issuecomment-525849264
Old memory icon: ⌨
New: ≡

frontend/apps/reader/modules/readerfooter.lua

@@ -53,7 +53,7 @@ local symbol_prefix = {
         book_time_to_read = "⏳",
         chapter_time_to_read = "⤻",
         frontlight = "☼",
-        mem_usage = "⌨",
+        mem_usage = "≡",
         wifi_status = "⚟",
     }
 }