pmc.perf.3: Mark only for amd64 architecture

Manual pages have an optional architecture specifier after the section
number in the document title macro. I highly recommend using it when
expedient, the extra information is available programmatically and it
uses no extra lines in the rendered page.

MFC after:		No, pmc.perf.3 is not MFCing
Reviewed by:		Anderson Nascimento <[email protected]>
Differential Revision:	https://reviews.freebsd.org/D60714

lib/libpmc/pmc.perf.3

@@ -4,7 +4,7 @@
 .\" SPDX-License-Identifier: BSD-2-Clause
 .\"
 .Dd October 13, 2026
-.Dt PMC.PERF 3
+.Dt PMC.PERF 3 amd64
 .Os
 .Sh NAME
 .Nm pmc.perf