txscript: Fix typo in README
txscript: Fix typo in README

Upstream commit 528ddaf23e0a68666a9992b81f978e49e2c61f5e.

jcmincke/dcrd,jrick/btcd,decred/dcrd,jcmincke/dcrd,jrick/dcrd,jrick/btcd,jrick/btcd,jcmincke/dcrd,decred/dcrd,jrick/dcrd