minor #10481 Update Multiple_kernels.rst (Alienpruts)

This PR was squashed before being merged into the 3.4 branch (closes #10481).

Discussion
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Update Multiple_kernels.rst

I would like to propose to change the following:

After a change in the classmap in composer.json, I would recommend using 'composer dumpautoload', instead of doing a 'composer install'.
The main reasoning behind this is that after a 'composer install' the parameters.yml file is regenerated, and your own parameters stored there are gone (learned this the hard way :) )

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Commits
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5cd7a3a Update Multiple_kernels.rst

Author: xabbuh

configuration/multiple_kernels.rst

@@ -119,7 +119,7 @@ to your ``composer.json`` autoload section:
         }
     }
 
-Then, run ``composer install`` to dump your new autoload config.
+Then, run ``composer dump-autoload`` to dump your new autoload config.
 
 Step 3) Define the Kernel Configuration
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~