math: change float64 bias constant from 1022 to 1023

This makes some subtle code easier to understand.

R=rsc
CC=golang-dev
https://golang.org/cl/3444043
diff --git a/src/pkg/math/bits.go b/src/pkg/math/bits.go
index d36cd18..1a97e76 100644
--- a/src/pkg/math/bits.go
+++ b/src/pkg/math/bits.go
@@ -10,7 +10,7 @@
 	uvneginf = 0xFFF0000000000000
 	mask     = 0x7FF
 	shift    = 64 - 11 - 1
-	bias     = 1022
+	bias     = 1023
 )
 
 // Inf returns positive infinity if sign >= 0, negative infinity if sign < 0.