minor #13151 Update mailer.rst (Yohann76)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Update mailer.rst

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Commits
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264d1d6 Update mailer.rst

Author: OskarStark

mailer.rst

@@ -576,7 +576,7 @@ encrypt a signed message and/or to sign an encrypted message.
 Before signing/encrypting messages, make sure to have:
 
 * The `OpenSSL PHP extension`_ properly installed and configured;
-* A valid S/MIME security certificate.
+* A valid `S/MIME`_ security certificate.
 
 Signing Messages
 ~~~~~~~~~~~~~~~~