minor #20332 [HttpClient] Rename addHeader to setHeader (MrYamous)

This PR was merged into the 7.1 branch.

Discussion
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[HttpClient] Rename addHeader to setHeader

Typo following #19339

Commits
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7fff76b rename addHeader to setHeader in httpclient

Author: javiereguiluz

http_client.rst

@@ -152,7 +152,7 @@ brings most of the available options with type-hinted getters and setters::
             ->setBaseUri('https://...')
             // replaces *all* headers at once, and deletes the headers you do not provide
             ->setHeaders(['header-name' => 'header-value'])
-            // set or replace a single header using addHeader()
+            // set or replace a single header using setHeader()
             ->setHeader('another-header-name', 'another-header-value')
             ->toArray()
     );