minor #14379 Configuration reference: Improve example of http_client::headers (Nyholm)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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Configuration reference: Improve example of http_client::headers

This will fix #14326

This can be merged to 4.4

Commits
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77f4d41 Configuration reference: Improve example of http_client::headers

Author: javiereguiluz

reference/configuration/framework.rst

@@ -873,7 +873,7 @@ headers
 **type**: ``array``
 
 An associative array of the HTTP headers added before making the request. This
-value must use the format ``['header-name' => header-value, ...]``.
+value must use the format ``['header-name' => 'value0, value1, ...']``.
 
 http_version
 ............