doc: prevent literal text block from being treated as non-literal one
This creates hg.1.html as expected.
--- a/hgext/histedit.py Mon Feb 01 22:06:35 2016 +0900
+++ b/hgext/histedit.py Mon Feb 01 22:06:35 2016 +0900
@@ -162,7 +162,7 @@
By default each edited revision needs to be present in histedit commands.
To remove revision you need to use ``drop`` operation. You can configure
-the drop to be implicit for missing commits by adding:
+the drop to be implicit for missing commits by adding::
[histedit]
dropmissing = True