msyscall.2: Use .Fn instead of .Nm in DESCRIPTION

[Josiah Frentsos]

Index: msyscall.2
===================================================================
RCS file: /cvs/src/lib/libc/sys/msyscall.2,v
retrieving revision 1.2
diff -u -p -r1.2 msyscall.2
--- msyscall.2  27 Nov 2019 20:53:05 -0000      1.2
+++ msyscall.2  14 Sep 2023 15:54:33 -0000
@@ -41,7 +41,7 @@ If
 is 0, no action is taken on the page that contains
 .Fa addr .
 .Pp
-.Nm
+.Fn msyscall
 is currently intended for use by
 .Xr ld.so 1
 only, and may be called only once to indicate the location of