Fix comment

Author: pitrou

cpp/src/arrow/testing/math.cc

@@ -27,8 +27,8 @@
 namespace arrow {
 namespace {
 
-// `kOneUlp` is equal to 1.0 - (1.0 - nextafter(1.0, -inf)).
-// That is, it is the delta between 1.0 and the FP value immediately before 1.0.
+// `kOneUlp` is equal to 1.0 - nextafter(1.0, -inf).
+// That is, it is the delta between 1.0 and the FP value immediately before it.
 // We're using this value because `frexp` returns a mantissa between 0.5 and 1.0.
 template <typename Float>
 constexpr Float kOneUlp;