minor #9815 Adding quotes to a server_version on an example just after explanation why are necessary (sergiosusa)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #9815).

Discussion
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Adding quotes to a server_version on an example just after explanation why are necessary

just adding some quotes to server_version, couple of lines before is explained this:

"Always wrap the server version number with quotes to parse it as a string instead of a float number"

Commits
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3db34d4 Update doctrine.rst

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -389,13 +389,13 @@ If you want to configure multiple connections in YAML, put them under the
                     user:             root
                     password:         null
                     host:             localhost
-                    server_version:   5.6
+                    server_version:   '5.6'
                 customer:
                     dbname:           customer
                     user:             root
                     password:         null
                     host:             localhost
-                    server_version:   5.7
+                    server_version:   '5.7'
 
 The ``database_connection`` service always refers to the *default* connection,
 which is the first one defined or the one configured via the