Update qtype_stack.php

Update fact sheet to give consistent notation.

Author: sangwinc

lang/en/qtype_stack.php

@@ -1651,7 +1651,7 @@
 Assume we have a function \(f(g(x))\), then defining \(u=g(x)\), the derivative with respect to \(x\) is given by:
 \[\frac{\mathrm{d}f(g(x))}{\mathrm{d}x} = \frac{\mathrm{d}g(x)}{\mathrm{d}x}\cdot\frac{\mathrm{d}f(u)}{\mathrm{d}u}.\]
 Alternatively, we can write:
-\[\frac{\mathrm{d}f(g(x))}{\mathrm{d}x} = f\'(g(x))\cdot g\'(x).\]
+\(f(g(x)))'{\mathrm{d}x} = f\'(g(x))\cdot g\'(x).\]
 ';
 
 $string['calc_rules_name'] = 'Calculus rules';