fix data headers

data.md

@@ -4,7 +4,7 @@ Low-level programming cares a lot about data layout. It's a big deal. It also pe
 influences the rest of the language, so we're going to start by digging into how data is
 represented in Rust.
 
-# The `rust` repr
+## The rust repr
 
 Rust gives you the following ways to lay out composite data:
 
@@ -124,7 +124,7 @@ In principle enums can use fairly elaborate algorithms to cache bits throughout
 with special constrained representations. As such it is *especially* desirable that we leave
 enum layout unspecified today.
 
-# Dynamically Sized Types (DSTs)
+## Dynamically Sized Types (DSTs)
 
 Rust also supports types without a statically known size. On the surface,
 this is a bit nonsensical: Rust must know the size of something in order to
@@ -212,12 +212,12 @@ struct Foo {
 For details as to *why* this is done, and how to make it not happen, check out
 [SOME OTHER SECTION].
 
-# Alternative representations
+## Alternative representations
 
 Rust allows you to specify alternative data layout strategies from the default Rust
 one.
 
-# repr(C)
+### repr(C)
 
 This is the most important `repr`. It has fairly simple intent: do what C does.
 The order, size, and alignment of fields is exactly what you would expect from
@@ -241,14 +241,14 @@ still consumes a byte of space.
 
 * This is equivalent to repr(u32) for enums (see below)
 
-# repr(packed)
+### repr(packed)
 
 `repr(packed)` forces rust to strip any padding it would normally apply.
 This may improve the memory footprint of a type, but will have negative
 side-effects from "field access is heavily penalized" to "completely breaks
 everything" based on target platform.
 
-# repr(u8), repr(u16), repr(u32), repr(u64)
+### repr(u8), repr(u16), repr(u32), repr(u64)
 
 These specify the size to make a c-like enum (one which has no values in its variants).