txscript: Fix typo. (#700)
txscript: Fix typo. (#700)

* Fix tiny typo. Bump copyright year.
* Clarify documentation.

davecgh/btcd,davecgh/btcd,jrick/btcd,jrick/btcd,jongillham/btcd,BitGo/prova,ltcsuite/ltcd,Stash-Crypto/btcd,Stash-Crypto/btcd,Roasbeef/btcd,dajohi/btcd,jcmincke/dcrd,Roasbeef/btcd,jcmincke/dcrd,ppcsuite/ppcd,decred/dcrd,cqswxj2013/btcd,dajohi/btcd,tb00/btcd,adiabat/btcd,BitGo/prova,aakselrod/btcd,dajohi/btcd,aakselrod/btcd,wallclockbuilder/btcd,jcmincke/dcrd,cqswxj2013/btcd,wallclockbuilder/btcd,jrick/dcrd,tb00/btcd,btcsuite/btcd,jrick/btcd,jrick/dcrd,ltcsuite/ltcd,stevenroose/btcd,jongillham/btcd,decred/dcrd,btcsuite/btcd,stevenroose/btcd,adiabat/btcd,ppcsuite/ppcd