[meltdown-mitigation] Be super explicit (#3501)

* [meltdown-mitigation] Be super explicit

It seems a few people get stuck on this (maybe because the sentence is quite long and for non-native speakers it's tough to parse - e.g. "plus or minus 10%" could feel confusing maybe?) so I thought maybe we become SUPER explicit.

* Update exercises/concept/meltdown-mitigation/.docs/instructions.md

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Co-authored-by: BethanyG <BethanyG@users.noreply.github.com>

exercises/concept/meltdown-mitigation/.docs/instructions.md

@@ -61,9 +61,7 @@ Implement the function called `fail_safe()`, which takes 3 parameters: `temperat
 - If `temperature * neutrons_produced_per_second` < 90% of `threshold`, output a status code of 'LOW'
   indicating that control rods must be removed to produce power.
 
-- If `temperature * neutrons_produced_per_second` are within plus or minus 10% of the `threshold`
-  the reactor is in _criticality_ and the status code of 'NORMAL' should be output, indicating that the
-  reactor is in optimum condition and control rods are in an ideal position.
+- If the value `temperature * neutrons_produced_per_second` is within 10% of the `threshold` (so either 0-10% less than the threshold, at the threshold, or 0-10% greater than the threshold), the reactor is in _criticality_ and the status code of 'NORMAL' should be output, indicating that the reactor is in optimum condition and control rods are in an ideal position.
 
 - If `temperature * neutrons_produced_per_second` is not in the above-stated ranges, the reactor is
   going into meltdown and a status code of 'DANGER' must be passed to immediately shut down the reactor.