rational-numbers: Remove redundant factor (#1727)

If `b₁ = 0`, then `r₁` is not a rational number anyway.

From https://github.com/exercism/problem-specifications/pull/1655/files#r421573667

exercises/rational-numbers/description.md

@@ -8,7 +8,7 @@ The difference of two rational numbers `r₁ = a₁/b₁` and `r₂ = a₂/b₂`
 
 The product (multiplication) of two rational numbers `r₁ = a₁/b₁` and `r₂ = a₂/b₂` is `r₁ * r₂ = (a₁ * a₂) / (b₁ * b₂)`.
 
-Dividing a rational number `r₁ = a₁/b₁` by another `r₂ = a₂/b₂` is `r₁ / r₂ = (a₁ * b₂) / (a₂ * b₁)` if `a₂ * b₁` is not zero.
+Dividing a rational number `r₁ = a₁/b₁` by another `r₂ = a₂/b₂` is `r₁ / r₂ = (a₁ * b₂) / (a₂ * b₁)` if `a₂` is not zero.
 
 Exponentiation of a rational number `r = a/b` to a non-negative integer power `n` is `r^n = (a^n)/(b^n)`.