Fix grammar-o in CONTRIBUTING

Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1625)

CONTRIBUTING

-HOW TO CONTRIBUTE TO PATCHES OpenSSL
+HOW TO CONTRIBUTE PATCHES TO OpenSSL
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 (Please visit https://www.openssl.org/community/getting-started.html for