Typo: 'a' to 'an' in destructors.md

Author: macaujack

src/destructors.md

@@ -284,7 +284,7 @@ An *extending pattern* is either
 * An [identifier pattern] that binds by reference or mutable reference.
 * A [struct][struct pattern], [tuple][tuple pattern], [tuple struct][tuple
   struct pattern], or [slice][slice pattern] pattern where at least one of the
-  direct subpatterns is a extending pattern.
+  direct subpatterns is an extending pattern.
 
 So `ref x`, `V(ref x)` and `[ref x, y]` are all extending patterns, but `x`,
 `&ref x` and `&(ref x,)` are not.