consistent section title casing

Author: chrissimpkins

src/part-1-intro.md

@@ -1,4 +1,4 @@
-# Part 1: Building, debugging, and contributing to Rustc
+# Part 1: Building, Debugging, and Contributing to `rustc`
 
 This section of the rustc-dev-guide contains knowledge that should be useful to you
 regardless of what part of the compiler you are working on. This includes both

src/part-2-intro.md

@@ -1,4 +1,4 @@
-# Part 2: How rustc works
+# Part 2: How `rustc` Works
 
 The remaining parts of this guide discuss how the compiler works. They go
 through everything from high-level structure of the compiler to how each stage

src/part-5-intro.md

@@ -1,4 +1,4 @@
-# From MIR to binaries
+# From MIR to Binaries
 
 All of the preceding chapters of this guide have one thing in common: we never
 generated any executable machine code at all! With this chapter, all of that