Update the location of the pod files

CLA: trivial
Since 99d63d4 ("Move manpages to man[1357] structure.", 2016-10-26), the location
of the pod files has changed.
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1951)

CONTRIBUTING

diff --git a/CONTRIBUTING b/CONTRIBUTING
index 1eebaf3..08c607a 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -50,5 +50,5 @@
     test/README for information on the test framework.
 
     6.  New features or changed functionality must include
-    documentation. Please look at the "pod" files in doc/apps, doc/crypto
-    and doc/ssl for examples of our style.
+    documentation. Please look at the "pod" files in doc/man[1357]
+    for examples of our style.