Fix typo in ch07

Change 'a output' to 'an output'

ch07.asciidoc

@@ -261,7 +261,7 @@ While nLocktime is a transaction level timelock, CLTV is an output based timeloc
 
 The +CLTV+ opcode takes one parameter as input, expressed as a number in the same format as nLocktime (either a block height or Unix epoch time). As indicated by the +VERIFY+ suffix, +CLTV+ is the type of opcode that halts execution of the script if the outcome is +FALSE+. If it results in TRUE, execution continues.
 
-In order to lock a output with +CLTV+, you insert it into the redeem script of the output, in the transaction that creates the output. For example, if Alice is paying Bob's address, the output would normally contain a P2PKH script like this:
+In order to lock an output with +CLTV+, you insert it into the redeem script of the output, in the transaction that creates the output. For example, if Alice is paying Bob's address, the output would normally contain a P2PKH script like this:
 
 ----
 DUP HASH160 <Bob's Public Key Hash> EQUALVERIFY