manifest-format: convert to markdown

The gitiles system doesn't render .txt files, so convert this to .md
for better display online.

Change-Id: Ie12e46daf008dd8c97ae2ffd21fb68bd948fe625
diff --git a/subcmds/manifest.py b/subcmds/manifest.py
index 5ceeb12..4a5228b 100644
--- a/subcmds/manifest.py
+++ b/subcmds/manifest.py
@@ -39,7 +39,7 @@
     helptext = self._helpDescription + '\n'
     r = os.path.dirname(__file__)
     r = os.path.dirname(r)
-    fd = open(os.path.join(r, 'docs', 'manifest-format.txt'))
+    fd = open(os.path.join(r, 'docs', 'manifest-format.md'))
     for line in fd:
       helptext += line
     fd.close()