minor #3929 update Blackfire documentation URL (Ca-Jou)

This PR was submitted for the 3.x branch but it was merged into the 2.x branch instead.

Discussion
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update Blackfire documentation URL

Update Blackfire documentation's URL in README file

Commits
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ed2cfbd update Blackfire documentation URL

README.rst

@@ -14,7 +14,7 @@ Sponsors
 
 .. raw:: html
 
-    <a href="https://blackfire.io/docs/introduction?utm_source=twig&utm_medium=github_readme&utm_campaign=logo">
+    <a href="https://docs.blackfire.io/introduction?utm_source=twig&utm_medium=github_readme&utm_campaign=logo">
         <img src="https://static.blackfire.io/assets/intemporals/logo/png/blackfire-io_secondary_horizontal_transparent.png?1" width="255px" alt="Blackfire.io">
     </a>