fix markup

Author: tshepang

src/doc/rustc-dev-guide/src/building/bootstrapping/writing-tools-in-bootstrap.md

@@ -3,17 +3,20 @@
 There are three types of tools you can write in bootstrap:
 
 - **`Mode::ToolBootstrap`**
+
   Use this for tools that don’t need anything from the in-tree compiler and can run with the stage0 `rustc`.
   The output is placed in the "bootstrap-tools" directory.
   This mode is for general-purpose tools built entirely with the stage0 compiler,
   including target libraries and only works for stage 0.
 
 - **`Mode::ToolStd`**
+
   Use this for tools that rely on the locally built std.
   The output goes into the "stageN-tools" directory.
   This mode is rarely used, mainly for `compiletest` which requires `libtest`.
 
 - **`Mode::ToolRustcPrivate`**
+
   Use this for tools that use the `rustc_private` mechanism,
   and thus depend on the locally built `rustc` and its rlib artifacts.
   This is more complex than the other modes because the tool must be built with the same compiler used for `rustc` and placed in the "stageN-tools" directory.