Style guide: Attempt to make the L2 and L3 headings more visually distinct

The default style of the L2 and L3 headers are too similar, making it difficult to see which is which.

BUG=none
NOTRY=true

Change-Id: I0deede2ad5766db9b63baa48be8e4da4aba784ca
Reviewed-on: https://chromium-review.googlesource.com/657697
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19759}

style-guide.md

diff --git a/style-guide.md b/style-guide.md
index 6f68ef6..1c81bb3 100644
--- a/style-guide.md
+++ b/style-guide.md
@@ -1,6 +1,6 @@
 # WebRTC coding style guide
 
-## General advice
+## **General advice**
 
 Some older parts of the code violate the style guide in various ways.
 
@@ -10,7 +10,7 @@
 * If making large changes to such code, consider first cleaning it up
   in a separate CL.
 
-## C++
+## **C++**
 
 WebRTC follows the [Chromium][chr-style] and [Google][goog-style] C++
 style guides. In cases where they conflict, the Chromium style guide
@@ -36,7 +36,7 @@
 
 See [the source](webrtc/api/array_view.h) for more detailed docs.
 
-## C
+## **C**
 
 There’s a substantial chunk of legacy C code in WebRTC, and a lot of
 it is old enough that it violates the parts of the C++ style guide
@@ -48,26 +48,26 @@
 * If making large changes to C code, consider converting the whole
   thing to C++ first.
 
-## Java
+## **Java**
 
 WebRTC follows the [Google Java style guide][goog-java-style].
 
 [goog-java-style]: https://google.github.io/styleguide/javaguide.html
 
-## Objective-C and Objective-C++
+## **Objective-C and Objective-C++**
 
 WebRTC follows the
 [Chromium Objective-C and Objective-C++ style guide][chr-objc-style].
 
 [chr-objc-style]: https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/objective-c/objective-c.md
 
-## Python
+## **Python**
 
 WebRTC follows [Chromium’s Python style][chr-py-style].
 
 [chr-py-style]: https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/styleguide.md#python
 
-## Build files
+## **Build files**
 
 The WebRTC build files are written in [GN][gn], and we follow
 the [Chromium GN style guide][chr-gn-style]. Additionally, there are