update markdown/help header format

Since gitiles recommends using # headers over ---/=== underlines,
change the manifest-format.md over and all our help texts.

Change-Id: I96391d41fba769e9f26870d497cf7cf01c8d8ab3
diff --git a/subcmds/branches.py b/subcmds/branches.py
index 2902684..fa1dff6 100644
--- a/subcmds/branches.py
+++ b/subcmds/branches.py
@@ -67,8 +67,7 @@
 
 Summarizes the currently available topic branches.
 
-Branch Display
---------------
+# Branch Display
 
 The branch display output by this command is organized into four
 columns of information; for example: