Formatting issues to correct in Dyalog v20.0

THIS IS BEING ADDED TO AS ISSUES ARE DISCOVERED – ANYONE CAN RESOLVE THEM AND MARK THEM AS DONE BUT PLEASE LEAVE THE ISSUE OPEN FOR NOW.

- [x] https://docs.dyalog.com/20.0/language-reference-guide/primitive-operators/power/
Reformat the admonition definition to conform to the standards so that it discplays as shown in the Conventions
- [x] Menu: Core Reference > Programming > Defined Functions and Operators > Traditional Functions and Operators > Control Structures
The menu item for "Leave statement" erroneously includes the right-hand side of the title bar (":Leave") – compare with all other control structure ToC entries.
- [x] https://docs.dyalog.com/20.0/programming-reference-guide/threads/multithreading-language-elements/
ToC entry does not match page title and has a rogue full stop!
- [x] https://docs.dyalog.com/20.0/unix-installation-and-configuration-guide/dyalog-serial-number/
Missing a space character between sentences: "is not permitted.For the full licence".
Extra space before the full stop: 'When you start a Session, your serial number is displayed in the banner ."
- [x] https://docs.dyalog.com/20.0/release-notes/announcements/#defining-dfns-with-unmatched-brackets
Correct the format of the admonition