Fix equation in mmme1029

mechanical/mmme1029_materials.md

@@ -39,7 +39,7 @@ tags: [ uni, nottingham, mmme1029, materials ]
 - Thermodynamics indicated that the higher the temperature, the greater the efficiency of heat to
 work:
 
-  $= \frac{T_{high}-T_{low}}{T_{high}}$ (in kelvin)
+  $$ \frac{ T_{high} - T_{low} }{ T_{high} } $$ where $T$ is in kelvin
 
 - The first steam engines were only 1% efficient, while modern steam engines are 35% efficient
   primarily due to improved high-temperature materials.