test/float_lit2: fix expressions in comment

The change corrects the values of the largest float32 value (f1) and the
value of the halfway point between f1 and the next, overflow value (f2).

Fixes #17012

Change-Id: Idaf9997b69d61fafbffdb980d751c9857732e14d
Reviewed-on: https://go-review.googlesource.com/29171Reviewed-by: Robert Griesemer <gri@golang.org>

test/float_lit2.go

@@ -13,12 +13,12 @@ import (
 	"math"
 )
 
-// The largest exact float32 is f₁ = (1+(1-2²³))×2¹²⁷ = (1-2²⁴)×2¹²⁸ = 2¹²⁸ - 2¹⁰⁴.
+// The largest exact float32 is f₁ = (1+1-1/2²³)×2¹²⁷ = (2-2⁻²³)×2¹²⁷ = 2¹²⁸ - 2¹⁰⁴.
 // The next float32 would be f₂ = (1+1)×2¹²⁷ = 1×2¹²⁸, except that exponent is out of range.
 // Float32 conversion rounds to the nearest float32, rounding to even mantissa:
 // between f₁ and f₂, values closer to f₁ round to f₁ and values closer to f₂ are rejected as out of range.
 // f₁ is an odd mantissa, so the halfway point (f₁+f₂)/2 rounds to f₂ and is rejected.
-// The halfway point is (f₁+f₂)/2 = 2¹²⁸ - 2¹⁰⁵.
+// The halfway point is (f₁+f₂)/2 = 2¹²⁸ - 2¹⁰³.
 //
 // The same is true of float64, with different constants: s/24/53/ and s/128/1024/.