Fix variable name typo from 'extension' to 'extensions' (#258)

This vector is called is the next section as 'extensions'

Author: oskarTom

en/03_Drawing_a_triangle/00_Setup/01_Instance.adoc

@@ -211,7 +211,7 @@ ignore for now.
 
 [,c++]
 ----
-auto extension = context.enumerateInstanceExtensionProperties();
+auto extensions = context.enumerateInstanceExtensionProperties();
 ----
 
 Each `VkExtensionProperties` struct contains the name and version of an