Fix spacing for blimp markdown.

The building page for blimp is missing a few newlines to ensure
that GN args and build commands are rendered correctly. This
CL adds the missing newlines.

BUG=None

Review URL: https://codereview.chromium.org/1356663003

Cr-Commit-Position: refs/heads/master@{#349984}

blimp/docs/build.md

@@ -5,9 +5,11 @@ be found in the GN [quick start guide](../../tools/gn/docs/quick_start.md).
 
 ## Android setup
 To setup GN, run the following command:
+
 ```
 gn args out-android/Debug
 ```
+
 This will bring up an editor, where you can type in the following:
 
 ```
@@ -21,10 +23,13 @@ use_goma = true
 
 ## Linux setup
 For building for Linux, you can have a side-by-side out-directory:
+
 ```
 gn args out-linux/Debug
 ```
+
 Use the same arguments as above, but remove `target_os`.
+
 ```
 is_debug = true
 is_clang = true