CodingGuidelines: fix subject-verb agreement

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: newren

Documentation/CodingGuidelines

@@ -688,7 +688,7 @@ For C programs:
          walk_dog(dog[1]);
 
    Cases where the array is employed as a whole rather than as its unit parts,
-   the plural forms is preferable. E.g:
+   the plural form is preferable. E.g:
 
          char *dogs[] = ...;
          walk_all_dogs(dogs);