unsafe: documentation typo.

Fixes #236.

R=r
CC=golang-dev
https://golang.org/cl/155072

src/pkg/unsafe/unsafe.go

@@ -30,7 +30,7 @@ func Sizeof(v ArbitraryType) int
 // number of bytes between the start of the struct and the start of the field.
 func Offsetof(v ArbitraryType) int
 
-// Alignof returns the alignment of the value v.  It is the minimum value m such
+// Alignof returns the alignment of the value v.  It is the maximum value m such
 // that the address of a variable with the type of v will always always be zero mod m.
 // If v is of the form obj.f, it returns the alignment of field f within struct object obj.
 func Alignof(v ArbitraryType) int