Use name attribute not id attribute for anchor tag in Markdown

Per dpranke, it seems Gerrit strips out <a id=foo> anchors.

BUG=

Review-Url: https://codereview.chromium.org/2567583002
Cr-Commit-Position: refs/heads/master@{#437673}

docs/linux_build_instructions.md

@@ -117,7 +117,7 @@ $ gn gen out/Default
 * For more info on GN, run `gn help` on the command line or read the
   [quick start guide](../tools/gn/docs/quick_start.md).
 
-### <a id=faster-builds></a>Faster builds
+### <a name="faster-builds"></a>Faster builds
 
 This section contains some things you can change to speed up your builds,
 sorted so that the things that make the biggest difference are first.