Fix markup for variable names (#9150)

Three references to `$isDevMode` were marked up with a single backtick, however two backticks are required in order for the variable name to be highlighted correctly (c.f. `ArrayCache`).

Author: pwaring

docs/en/reference/configuration.rst

@@ -85,9 +85,9 @@ Or if you prefer YAML:
 
 Inside the ``Setup`` methods several assumptions are made:
 
--  If `$isDevMode` is true caching is done in memory with the ``ArrayCache``. Proxy objects are recreated on every request.
--  If `$isDevMode` is false, check for Caches in the order APC, Xcache, Memcache (127.0.0.1:11211), Redis (127.0.0.1:6379) unless `$cache` is passed as fourth argument.
--  If `$isDevMode` is false, set then proxy classes have to be explicitly created through the command line.
+-  If ``$isDevMode`` is true caching is done in memory with the ``ArrayCache``. Proxy objects are recreated on every request.
+-  If ``$isDevMode`` is false, check for Caches in the order APC, Xcache, Memcache (127.0.0.1:11211), Redis (127.0.0.1:6379) unless `$cache` is passed as fourth argument.
+-  If ``$isDevMode`` is false, set then proxy classes have to be explicitly created through the command line.
 -  If third argument `$proxyDir` is not set, use the systems temporary directory.
 
 If you want to configure Doctrine in more detail, take a look at the :doc:`Advanced Configuration <reference/advanced-configuration>` section.