Replace use of mathematical minus with an em-dash (from tb@) and break up

a long sentence (from tj@).

src/lib/libc/hash/md5.3

@@ -16,9 +16,9 @@
 .\" If we meet some day, and you think this stuff is worth it, you
 .\" can buy me a beer in return. Poul-Henning Kamp
 .\"
-.\" 	$OpenBSD: md5.3,v 1.5 2015/11/10 23:48:18 jmc Exp $
+.\" 	$OpenBSD: md5.3,v 1.6 2017/02/23 20:29:17 daniel Exp $
 .\"
-.Dd $Mdocdate: November 10 2015 $
+.Dd $Mdocdate: February 23 2017 $
 .Dt MD5INIT 3
 .Os
 .Sh NAME
@@ -67,9 +67,9 @@ MD5 has been broken; it should only be used where necessary for
 backward compatibility.
 The attack on MD5 is in the nature of finding
 .Dq collisions
-\- that is, multiple
-inputs which hash to the same value; it is still unlikely for an attacker
-to be able to determine the exact original input given a hash value.
+\(em that is, multiple inputs which hash to the same value.
+It is still unlikely for an attacker to be able to determine the exact
+original input given a hash value.
 .Pp
 The
 .Fn MD5Init ,