help: rename the Format section to Structure

help: rename the Format section to Structure

This reduces the confusion from `hg help config.format` which deals with
repository format. (But also profiling.format, and progress.format)

--- a/mercurial/help/config.txt	Wed Dec 30 17:04:08 2015 +0000
+++ b/mercurial/help/config.txt	Wed Dec 30 17:10:56 2015 +0000
@@ -11,8 +11,8 @@
 See :hg:`help config.syntax` and :hg:`help config.files`
 for information about how and where to override things.
 
-Format
-======
+Structure
+=========
 
 The configuration files use a simple ini-file format. A configuration
 file consists of sections, led by a ``[section]`` header and followed