Add missing .pod extension to EVP_PKEY_CTX_set_tls1_prf_md

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit efba60ca7ab72cae62baad2aaaf2da32d1093c38)

diff --git a/doc/crypto/EVP_PKEY_CTX_set_tls1_prf_md b/doc/crypto/EVP_PKEY_CTX_set_tls1_prf_md.pod
similarity index 100%
rename from doc/crypto/EVP_PKEY_CTX_set_tls1_prf_md
rename to doc/crypto/EVP_PKEY_CTX_set_tls1_prf_md.pod