update comment

types/context.go

@@ -108,7 +108,7 @@ func (c Context) TxCount() int                                  { return c.txCou
 func (c Context) BlockGasUsed() uint64                          { return c.blockGasUsed }
 func (c Context) BlockGasWanted() uint64                        { return c.blockGasWanted }
 
-// clone the header before returning
+// BlockHeader returns the header by value (shallow copy).
 func (c Context) BlockHeader() cmtproto.Header {
 	return c.header
 }