tsort.1: use mandoc literal block for example
diff --git a/tsort.1 b/tsort.1
index 27ec4f2..2f8f0a1 100644
--- a/tsort.1
+++ b/tsort.1
@@ -31,6 +31,7 @@
 An error occurred.
 .El
 .Sh EXAMPLES
+.Bd -literal -offset left
 The input
 
     a a
@@ -63,7 +64,7 @@
     ┏━━━┓    ┏━━━┓
     ┃ e ┃───→┃ f ┃
     ┗━━━┛    ┗━━━┛
-
+.Ed
 .Sh STANDARDS
 The
 .Nm