Don't refer to pppd(8).

usr.sbin/ppp/ppp.8.m4

@@ -44,10 +44,10 @@ changecom(,)dnl
 This is a user process
 .Em PPP
 software package.
-Normally,
+Sometimes,
 .Em PPP
 is implemented as a part of the kernel (e.g., as managed by
-.Xr pppd 8 )
+.Nm pppd )
 and it is thus somewhat hard to debug and/or modify its behaviour.
 However, in this implementation
 .Em PPP
@@ -2701,7 +2701,7 @@ program.
 Note: There is a problem negotiating
 .Ar deflate
 capabilities with
-.Xr pppd 8
+.Nm pppd
 - a
 .Em PPP
 implementation available under many operating systems.
@@ -2734,7 +2734,7 @@ Default: Disabled and Denied.
 This is a variance of the
 .Ar deflate
 option, allowing negotiation with the
-.Xr pppd 8
+.Nm pppd
 program.
 Refer to the
 .Ar deflate
@@ -2882,7 +2882,7 @@ acts as the authenticatee with both protocols
 the protocols are used alternately in response to challenges.
 .Pp
 Note: If only LANMan is enabled,
-.Xr pppd 8
+.Nm pppd
 (version 2.3.5) misbehaves when acting as authenticatee.
 It provides both
 the NT and the LANMan answers, but also suggests that only the NT answer
@@ -6091,7 +6091,6 @@ ifdef({LOCALRAD},{},{.Xr libradius 3 ,
 .Xr named 8 ,
 .Xr ping 8 ,
 .Xr pppctl 8 ,
-.Xr pppd 8 ,
 .Xr pppoed 8 ,
 .Xr route 8 ,
 .Xr sshd 8 ,