minor symfony#4709 [Reference] fix wording (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Reference] fix wording

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | symfony#4673

Commits
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10b06da [Reference] fix wording

Author: wouterj

reference/configuration/framework.rst

@@ -530,7 +530,7 @@ cache
 **type**: ``string``
 
 This value is used to determine the service that is used to persist class
-metadata in a cache. The actual service name is built by prefix the configured
+metadata in a cache. The actual service name is built by prefixing the configured
 value with ``validator.mapping.cache.`` (e.g. if the value is ``apc``, the
 ``validator.mapping.cache.apc`` service will be injected). The service has
 to implement the :class:`Symfony\\Component\\Validator\\Mapping\\Cache\\CacheInterface`.