examples/mdoc: Remove document description quotes

Quotes in roff do not work the way quotes in shell do,
remove them from the examples to hopefully lead less people astray.

MFC after:	3 days

share/examples/mdoc/example.1

@@ -10,7 +10,7 @@
 .Os
 .Sh NAME
 .Nm example
-.Nd "example command manual page"
+.Nd example command manual page
 .Sh SYNOPSIS
 .Nm
 .Op Fl abc

share/examples/mdoc/example.3

@@ -10,7 +10,7 @@
 .Os
 .Sh NAME
 .Nm example
-.Nd "example library function manual page"
+.Nd example library function manual page
 .Sh LIBRARY
 .\" Note: list of available libraries is available in mdoc(7)
 .Lb libc

share/examples/mdoc/example.4

@@ -10,7 +10,7 @@
 .Os
 .Sh NAME
 .Nm example
-.Nd "example device driver manual page"
+.Nd example device driver manual page
 .Sh SYNOPSIS
 To compile the driver into the kernel,
 place the following lines in the

share/examples/mdoc/example.9

@@ -10,7 +10,7 @@
 .Os
 .Sh NAME
 .Nm example
-.Nd "example kernel interface manual page"
+.Nd example kernel interface manual page
 .Sh SYNOPSIS
 .In sys/example.h
 .Ft int