bug #6757 Fix typo in external_parameters.rst (gmorel)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6757).

Discussion
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Fix typo in external_parameters.rst

Missing `:` in yml doc

Keep up the good work !

Commits
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2739a9c Fix typo in external_parameters.rst

Author: weaverryan

configuration/external_parameters.rst

@@ -68,7 +68,7 @@ You can now reference these parameters wherever you need them.
 
         doctrine:
             dbal:
-                driver    pdo_mysql
+                driver:   pdo_mysql
                 dbname:   symfony_project
                 user:     '%database.user%'
                 password: '%database.password%'