Merge #135: remove orphan heading "Locking and unlocking UTXOs"

f693c16 remove hanging headling (Simon Tennant)

Pull request description:

  While I'd love to know what was going to be added here, it's better that we remove the headline until we have content for it.

ACKs for top commit:
  uncomputable:
    ACK f693c16

Tree-SHA512: a05a451a1ad54cfd7ffd5b13494686e61d542975dc0d2adc6fe705d1c55f25357eff533c8bbbfd4af5114e8253f4cd9c3bea09cc536f0a646a4e3f80fe8efc80

Author: uncomputable

book/src/SUMMARY.md

@@ -12,5 +12,3 @@
 - [Match Expression](./match_expression.md)
 - [Functions](./function.md)
 - [Programs](./program.md)
-
-# Locking and Unlocking UTXOs