Fix a code comment in Upload->_file_mime_type()

Availability of dangerous functions is now tested using function_usable().
diff --git a/system/libraries/Upload.php b/system/libraries/Upload.php
index acd76b0..1c14f99 100644
--- a/system/libraries/Upload.php
+++ b/system/libraries/Upload.php
@@ -1212,7 +1212,7 @@
 		 * Notes:
 		 *	- the DIRECTORY_SEPARATOR comparison ensures that we're not on a Windows system
 		 *	- many system admins would disable the exec(), shell_exec(), popen() and similar functions
-		 *	  due to security concerns, hence the function_exists() checks
+		 *	  due to security concerns, hence the function_usable() checks
 		 */
 		if (DIRECTORY_SEPARATOR !== '\\')
 		{
@@ -1223,7 +1223,7 @@
 			if (function_usable('exec'))
 			{
 				/* This might look confusing, as $mime is being populated with all of the output when set in the second parameter.
-				 * However, we only neeed the last line, which is the actual return value of exec(), and as such - it overwrites
+				 * However, we only need the last line, which is the actual return value of exec(), and as such - it overwrites
 				 * anything that could already be set for $mime previously. This effectively makes the second parameter a dummy
 				 * value, which is only put to allow us to get the return status code.
 				 */