[Meltdown Mitigation]: Corrected "critical" to "balanced in criticality" on line 15 of Instructions (exercism#3993)

* Corrected critical to balanced in criticaltiy on line 15 of intructions.

* Corrected docstring to refer to balanced in criticality.

Author: BethanyG

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

@@ -11,8 +11,8 @@ The following three tasks are all related to writing code for maintaining ideal
 
 ## 1. Check for criticality
 
-The first thing a control system has to do is check if the reactor is balanced in criticality.
-A reactor is said to be critical if it satisfies the following conditions:
+The first thing a control system has to do is check if the reactor is _balanced in criticality_.
+A reactor is said to be balanced in criticality if it satisfies the following conditions:
 
 - The temperature is less than 800 K.
 - The number of neutrons emitted per second is greater than 500.

exercises/concept/meltdown-mitigation/conditionals.py

@@ -8,7 +8,7 @@ def is_criticality_balanced(temperature, neutrons_emitted):
     :param neutrons_emitted: int or float - number of neutrons emitted per second.
     :return: bool - is criticality balanced?
 
-    A reactor is said to be critical if it satisfies the following conditions:
+    A reactor is said to be balanced in criticality if it satisfies the following conditions:
     - The temperature is less than 800 K.
     - The number of neutrons emitted per second is greater than 500.
     - The product of temperature and neutrons emitted per second is less than 500000.