man: Use major.minor version for .Nx macros

The macro only recognizes this version form, and not just major alone.
Signed-off-by: Guillem Jover <guillem@hadrons.org>

man/strtoi.3bsd

@@ -229,7 +229,7 @@ extension.
 The
 .Fn strtoi
 function first appeared in
-.Nx 7 .
+.Nx 7.0 .
 .Ox
 introduced the
 .Fn strtonum 3bsd

man/strtonum.3bsd

@@ -151,7 +151,7 @@ function first appeared in
 .Ox 3.6 .
 .Fn strtonum
 was redesigned in
-.Nx 8
+.Nx 8.0
 as
 .Fn strtoi 3bsd
 and

man/strtou.3bsd

@@ -229,7 +229,7 @@ extension.
 The
 .Fn strtou
 function first appeared in
-.Nx 7 .
+.Nx 7.0 .
 .Ox
 introduced the
 .Fn strtonum 3bsd