minor #9227 Updated a diagram to SVG format (javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
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Updated a diagram to SVG format

I was going to work on #8903 ... but I thought that we could finally work on moving to SVG-based diagrams. So this is a real test before switching all diagrams to SVG.

Commits
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bdb2a6c Updated a diagram to SVG format

Author: javiereguiluz

_images/security/authentication-guard-methods.png

@@ -432,7 +432,7 @@ Each authenticator needs the following methods:
 
 The picture below shows how Symfony calls Guard Authenticator methods:
 
-.. image:: /_images/security/authentication-guard-methods.png
+.. image:: /_images/security/authentication-guard-methods.svg
    :align: center
 
 .. _guard-customize-error: