Fix a grammar issue in the shape performance warning message

shape.c

@@ -734,7 +734,7 @@ rb_shape_get_next(rb_shape_t *shape, VALUE obj, ID id)
                     rb_category_warn(
                         RB_WARN_CATEGORY_PERFORMANCE,
                         "The class %"PRIsVALUE" reached %d shape variations, instance variables accesses will be slower and memory usage increased.\n"
-                        "It is recommended to define instance variable in a consistent order, for instance by eagerly defining them all in the #initialize method.",
+                        "It is recommended to define instance variables in a consistent order, for instance by eagerly defining them all in the #initialize method.",
                         rb_class_path(klass),
                         SHAPE_MAX_VARIATIONS
                     );