minor #15477 Minor clarification (ThomasLandauer)

This PR was merged into the 4.4 branch.

Discussion
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Minor clarification

The text above says:
> By default the first transport is used

The wording I changed suggested that the first transport called "main" is used (not the very first one in total).

Commits
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3cd1e7a Minor clarification

Author: javiereguiluz

mailer.rst

@@ -948,10 +948,10 @@ By default the first transport is used. The other transports can be selected by
 adding an ``X-Transport`` header (which Mailer will remove automatically from
 the final email)::
 
-    // Send using first "main" transport ...
+    // Send using first transport ("main"):
     $mailer->send($email);
 
-    // ... or use the "alternative" one
+    // ... or use the transport "alternative":
     $email->getHeaders()->addTextHeader('X-Transport', 'alternative');
     $mailer->send($email);