minor #14237 Update http_client.rst (ThomasLandauer)

This PR was merged into the 4.4 branch.

Discussion
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Update http_client.rst

Integrating the findings of symfony/symfony#38082 (comment)

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c8e5dd6 Update http_client.rst

Author: OskarStark

http_client.rst

@@ -637,6 +637,11 @@ when the streams are large)::
         'body' => $formData->bodyToString(),
     ]);
 
+If you need to add a custom HTTP header to the upload, you can do::
+
+    $headers = $formData->getPreparedHeaders()->toArray();
+    $headers[] = 'X-Foo: bar';
+
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