Fix typo in Feature Policy for idle-detection

As listed in MDN, the policy is spelled as `idle-detection`, and not as `idle_detection`. Hence, browsers were unable to process the policy correctly, effectively ignoring it.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Permissions-Policy/idle-detection

Author: MrSerth

actionpack/lib/action_dispatch/http/permissions_policy.rb

@@ -98,7 +98,7 @@ def permissions_policy=(policy)
       geolocation:          "geolocation",
       gyroscope:            "gyroscope",
       hid:                  "hid",
-      idle_detection:       "idle_detection",
+      idle_detection:       "idle-detection",
       magnetometer:         "magnetometer",
       microphone:           "microphone",
       midi:                 "midi",