docs: fix text decoration

Author: kenjis

user_guide_src/source/libraries/caching.rst

@@ -38,8 +38,8 @@ Available names are: dummy, file, memcached, redis, predis, wincache.
 $backupHandler
 --------------
 
-In the case that the first choice $handler is not available, this is the next cache handler to load.
-This is commonly the **file** handler since the file system is always available, but may not fit
+In the case that the first choice ``$handler`` is not available, this is the next cache handler to load.
+This is commonly the ``File`` handler since the file system is always available, but may not fit
 more complex, multi-server setups.
 
 $prefix