Fix heading levels in the query chapter (#1305)

This should fix the incorrect TOC rendering in #1303.

src/query.md

@@ -43,7 +43,7 @@ compiler (for example, generating [MIR](./mir/)) currently work exactly like thi
 in-depth description of what queries are and how they work.
 If you intend to write a query of your own, this is a good read.
 
-### Invoking queries
+## Invoking queries
 
 Invoking a query is simple. The [`TyCtxt`] ("type context") struct offers a method
 for each defined query. For example, to invoke the `type_of`
@@ -55,7 +55,7 @@ let ty = tcx.type_of(some_def_id);
 
 [`TyCtxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html
 
-### How the compiler executes a query
+## How the compiler executes a query
 
 So you may be wondering what happens when you invoke a query
 method. The answer is that, for each query, the compiler maintains a
@@ -64,7 +64,7 @@ simple: we clone the return value out of the cache and return it
 (therefore, you should try to ensure that the return types of queries
 are cheaply cloneable; insert an `Rc` if necessary).
 
-#### Providers
+### Providers
 
 If, however, the query is *not* in the cache, then the compiler will
 try to find a suitable **provider**. A provider is a function that has
@@ -97,7 +97,7 @@ fn provider<'tcx>(
 Providers take two arguments: the `tcx` and the query key.
 They return the result of the query.
 
-####  How providers are setup
+###  How providers are setup
 
 When the tcx is created, it is given the providers by its creator using
 the [`Providers`][providers_struct] struct. This struct is generated by
@@ -164,7 +164,7 @@ they define both a `provide` and a `provide_extern` function, through
 [rustc_metadata]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html
 [wasm_import_module_map]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/back/symbol_export/fn.wasm_import_module_map.html
 
-### Adding a new query
+## Adding a new query
 
 How do you add a new query?
 Defining a query takes place in two steps:
@@ -238,7 +238,7 @@ which is used to cheaply modify MIR in place. See the definition
 of `Steal` for more details. New uses of `Steal` should **not** be
 added without alerting `@rust-lang/compiler`.
 
-#### Query structs and descriptions
+### Query structs and descriptions
 
 For each query, the `rustc_queries` macro will generate a "query struct"
 named after the query. This struct is a kind of placeholder
@@ -294,7 +294,7 @@ rustc_queries! {
 
 `rustc_queries` macro will generate an appropriate `impl` automatically.
 
-## External Links
+## External links
 
 Related design ideas, and tracking issues: