Fix the names of table of contents and heading are different

Not to write the part of Appendix `A` in heading along with other appendix.

Author: giraffate

src/appendix/background.md

@@ -1,4 +1,4 @@
-# Appendix B: Background topics
+# Background topics
 
 This section covers a numbers of common compiler terms that arise in
 this guide. We try to give the general definition while providing some

src/appendix/code-index.md

@@ -1,4 +1,4 @@
-# Appendix D: Code Index
+# Code Index
 
 rustc has a lot of important data structures. This is an attempt to give some
 guidance on where to learn more about some of the key data structures of the

src/appendix/glossary.md

@@ -1,4 +1,4 @@
-# Appendix C: Glossary
+# Glossary
 
 Term                                     | Meaning
 -----------------------------------------|--------