Correct epdConfig mode to mono

Author: tyeth

components/display/eink-29-flexible-monochrome-296x128/definition.json

@@ -12,7 +12,7 @@
             "bus": 0
         },
         "epdConfig": {
-            "mode": "monochrome",
+            "mode": "mono",
             "width": 296,
             "height": 128
         }

components/display/eink-37-monochrome-416x240/definition.json

@@ -12,7 +12,7 @@
             "bus": 0
         },
         "epdConfig": {
-            "mode": "monochrome",
+            "mode": "mono",
             "width": 416,
             "height": 240
         }

components/display/eink-583-monochrome-648x480/definition.json

@@ -12,7 +12,7 @@
             "bus": 0
         },
         "epdConfig": {
-            "mode": "monochrome",
+            "mode": "mono",
             "width": 648,
             "height": 480
         }