Revise keys in number-play-strings_en.json

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From the CRC:

Make sure the string keys are all perfect, because they are difficult to change after 1.0.0 is published. Guidelines for string keys are:

(1) Strings keys should generally match their values. E.g.:

"helloWorld": {
  value: "Hello World!"
},
"quadraticTerms": {
  value: "Quadratic Terms"
}

(2) If a string key would be exceptionally long, use a key name that is an abbreviated form of the string value, or that captures the purpose/essence of the value.

Inspecting number-play_en.json, these strings do not conform. They should be "hearTotal" and "hearTotalDescription".

  "readAloud": {
    "value": "Hear Total"
  },
  "readAloudDescription": {
    "value": "Hear the updated total as you interact."
  },

It would be easy for me to change these, but I'm not sure if there are existing translations for this sim that would be disturbed (prototype?), or if it's OK to disturb prototype strings. Even if there are prototype translations, I recommend that we still make this change.

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Duplicate of #206, closing.