getopt_long.3: Use "optstring" consistently

The SYNOPSIS uses "optstring" as the argument name but the body text
used a mixture of "optstr" and "optstring."

Sponsored by:	The FreeBSD Foundation

lib/libc/stdlib/getopt_long.3

@@ -217,7 +217,7 @@ an error and return
 Specifying
 .Ql \&:
 in
-.Fa optstr
+.Fa optstring
 will cause the error message to be suppressed and
 .Ql \&:
 to be returned instead.
@@ -229,7 +229,7 @@ a leading
 or
 .Ql \&-
 in
-.Fa optstr
+.Fa optstring
 also has special meaning.
 If either of these are specified, they must appear before
 .Ql \&: .