sync/atomic: fix comment referencing Value.Store's argument name

Fixes #9029

LGTM=adg, r
R=r, adg
CC=golang-codereviews
https://golang.org/cl/161630044

src/sync/atomic/value.go

@@ -38,7 +38,7 @@ func (v *Value) Load() (x interface{}) {
 	return
 }
 
-// Store sets the value of the Value to v.
+// Store sets the value of the Value to x.
 // All calls to Store for a given Value must use values of the same concrete type.
 // Store of an inconsistent type panics, as does Store(nil).
 func (v *Value) Store(x interface{}) {