Fixed ToC chapter titles

book/src/06_ticket_management/00_intro.md

@@ -1,4 +1,4 @@
-# Intro
+# Ticket Management
 
 In the previous chapter we modelled `Ticket` in a vacuum: we defined its fields and their constraints, we learned
 how to best represent them in Rust, but we didn't consider how `Ticket` fits into a larger system.

book/src/07_threads/00_intro.md

@@ -1,4 +1,4 @@
-# Intro
+# Threads
 
 One of Rust's big promises is _fearless concurrency_: making it easier to write safe, concurrent programs.
 We haven't seen much of that yet. All the work we've done so far has been single-threaded.