unamend: fix command summary line

Before this patch, the docstring started with a newline, which led the
summary line (shown in e.g. `hg help -c`) to be blank.

Differential Revision: https://phab.mercurial-scm.org/D1943

hgext/uncommit.py

@@ -193,8 +193,7 b' def predecessormarkers(ctx):'
 
 @command('^unamend', [])
 def unamend(ui, repo, **opts):
-    """
-    undo the most recent amend operation on a current changeset
+    """undo the most recent amend operation on a current changeset
 
     This command will roll back to the previous version of a changeset,
     leaving working directory in state in which it was before running