Fix typo (missing negation) (#242)

Author: marcospgp

optimization-2.md

@@ -142,7 +142,7 @@ f_a(x) = ax
 \frac{df}{dx} = a
 $$
 
-Where the functions \\(f_c, f_a\\) translate the input by a constant of \\(c\\) and scale the input by a constant of \\(a\\), respectively. These are technically special cases of addition and multiplication, but we introduce them as (new) unary gates here since we do need the gradients for the constants. \\(c,a\\). The full circuit then looks as follows:
+Where the functions \\(f_c, f_a\\) translate the input by a constant of \\(c\\) and scale the input by a constant of \\(a\\), respectively. These are technically special cases of addition and multiplication, but we introduce them as (new) unary gates here since we do not need the gradients for the constants. \\(c,a\\). The full circuit then looks as follows:
 
 <div class="fig figleft fighighlight">
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