Update E0164.md

Author: Dylan-DPC

src/librustc_error_codes/error_codes/E0164.md

@@ -22,7 +22,7 @@ fn bar(foo: A) {
 ```
 
 This error means that an attempt was made to match something which is neither a
-tuple struct nor a tuple variant. Only these two elements are allowed as
+tuple struct nor a tuple variant. Only these two elements are allowed as a
 pattern:
 
 ```