docs: add default value for karma watch option

(cherry picked from commit a3b676f)

Author: alan-agius4

packages/angular_devkit/build_angular/src/karma/schema.json

@@ -103,7 +103,8 @@
     },
     "watch": {
       "type": "boolean",
-      "description": "Run build when files change."
+      "description": "Run build when files change.",
+      "default": true
     },
     "poll": {
       "type": "number",