limits: Improve consistency

Historical precedent seems pretty consistent: size limits have singular
names, number limits have plural names.  RLIMIT_VMM broke this, and I
made matters worse by referring to this limit as “vmms” in limits(1).
Consistently use “vms” everywhere user-visible, while leaving the
question of whether or not to rename RLIMIT_VMM itself for another day.

Fixes:		1092ec8b33 ("kern: Introduce RLIMIT_VMM")
Fixes:		53af2026f2 ("limits: Unbreak after RLIMIT_VMM addition")
Reviewed by:	bnovkov
Differential Revision:	https://reviews.freebsd.org/D57265

sys/sys/resource.h

@@ -145,7 +145,7 @@ static const char *rlimit_ident[] = {
 	"kqueues",
 	"umtx",
 	"pipebuf",
-	"vmm",
+	"vms",
 };
 #endif
 

usr.bin/limits/limits.c

@@ -247,7 +247,7 @@ static struct {
     { "kqueues",	login_getcapnum  },
     { "umtxp",		login_getcapnum  },
     { "pipebuf",	login_getcapnum  },
-    { "vmms",		login_getcapnum  },
+    { "vms",		login_getcapnum  },
 };
 
 _Static_assert(nitems(resources) == RLIM_NLIMITS,