Tweak some page titles

doc/03-cli.md

@@ -1,4 +1,4 @@
-# Command-line interface
+# Command-line interface / Commands
 
 You've already learned how to use the command-line interface to do some
 things. This chapter documents all the available commands.

doc/04-schema.md

@@ -1,4 +1,4 @@
-# composer.json
+# The composer.json Schema
 
 This chapter will explain all of the fields available in `composer.json`.