[ci skip] Merge pull request #4986 from ka7/feature/spelling

Spelling fixes in comment blocks and docs
diff --git a/system/libraries/Cache/drivers/Cache_memcached.php b/system/libraries/Cache/drivers/Cache_memcached.php
index 17e3611..b642a2c 100644
--- a/system/libraries/Cache/drivers/Cache_memcached.php
+++ b/system/libraries/Cache/drivers/Cache_memcached.php
@@ -110,7 +110,7 @@
 
 			if ($this->_memcached instanceof Memcache)
 			{
-				// Third parameter is persistance and defaults to TRUE.
+				// Third parameter is persistence and defaults to TRUE.
 				$this->_memcached->addServer(
 					$cache_server['hostname'],
 					$cache_server['port'],