Fix hidden Z on Hands Down Promethium matrix (#401)

Closes #400.

This is based on how the Canary matrix JSON file handles BracketLeft, BracketRight, Backslash, and Minus, Equals.

Author: peterjc

packages/keybr-generators/layouts/en_hands_down_promethium.json

@@ -10,8 +10,8 @@
   "Digit8": "8*",
   "Digit9": "9(",
   "Digit0": "0)",
-  "Minus": "]}",
-  "Equal": "[{",
+  "Minus": "-_",
+  "Equal": "=+",
   "KeyQ": "fF",
   "KeyW": "pP",
   "KeyE": "dD",
@@ -22,9 +22,9 @@
   "KeyI": "oO",
   "KeyO": "yY",
   "KeyP": "bB",
-  "BracketLeft": "zZ",
-  "BracketRight": "",
-  "Backslash": ";:",
+  "BracketLeft": "[{",
+  "BracketRight": "]}",
+  "Backslash": "zZ",
   "KeyA": "sS",
   "KeyS": "nN",
   "KeyD": "tT",