go/constant: fix doc strings

Fixes #14357.

Change-Id: I91acff0b0cc7be2bcbad68925a19a437dbd4c83d
Reviewed-on: https://go-review.googlesource.com/19620Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/constant/value.go

@@ -276,10 +276,10 @@ func smallRat(x *big.Float) bool {
 // MakeUnknown returns the Unknown value.
 func MakeUnknown() Value { return unknownVal{} }
 
-// MakeBool returns the Bool value for x.
+// MakeBool returns the Bool value for b.
 func MakeBool(b bool) Value { return boolVal(b) }
 
-// MakeString returns the String value for x.
+// MakeString returns the String value for s.
 func MakeString(s string) Value { return stringVal(s) }
 
 // MakeInt64 returns the Int value for x.