minor #21466 Fix RST in igbinary in the cache documentation (alexander-schranz)

This PR was merged into the 6.4 branch.

Discussion
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 Fix RST in igbinary in the cache documentation

```diff
-``igbinary`
+``igbinary``
```

Commits
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9d37a1a Fix rst in igbinary in the cache documentation

Author: javiereguiluz

components/cache/adapters/redis_adapter.rst

@@ -305,7 +305,7 @@ Encrypts cached data using Sodium for enhanced security::
 DefaultMarshaller with igbinary Serialization
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Uses ``igbinary` for faster and more efficient serialization when available::
+Uses ``igbinary`` for faster and more efficient serialization when available::
 
     $marshaller = new DefaultMarshaller(true);