command: fix default show-text duration param value

The mpv manpage seems to be wrong here. A look at mpv's command.c show
that -1 works fine here.

1 files changed, 1 insertions, 1 deletions

diff --git a/mpv.py b/mpv.py
index cd3fc3b..831a813 100644
--- a/mpv.py
+++ b/mpv.py
@@ -755,7 +755,7 @@ class MPV(object):
         """Mapped mpv osd command, see man mpv(1)."""
         self.command('osd')
 
-    def show_text(self, string, duration='-', level=None):
+    def show_text(self, string, duration='-1', level=None):
         """Mapped mpv show_text command, see man mpv(1)."""
         self.command('show_text', string, duration, level)