doc: reflog: fix drop subheading

The tilde (~) count doesn’t match the length of the heading.  In turn
you get a bunch of `<sub>~</sub>` instead of the intended `<h3>` in the
HTML output.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-reflog.adoc

@@ -138,7 +138,7 @@ Options for `delete`
 used with `expire`.
 
 Options for `drop`
-~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~
 
 --all::
 	Drop the reflogs of all references from all worktrees.