Fix typo in NOTES.PERL

CLA: trivial

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8905)

NOTES.PERL

@@ -78,7 +78,7 @@
  ---------------------------------
 
  There are a number of ways to install a perl module.  In all
- descriptions below, Text::Template will server as an example.
+ descriptions below, Text::Template will serve as an example.
 
  1. for Linux users, the easiest is to install with the use of your
     favorite package manager.  Usually, all you need to do is search