Fix Grammar

Fix minor grammar error in comment. No source code was changed.

Before:
```
 /**
     * Implements a circle, or spherical, geometry for an Body with
     * radius.
     *
     * @class Circle
     * @extends Body
     * @constructor
     */
```

After:
```
 /**
     * Implements a circle, or spherical, geometry for a Body with
     * radius.
     *
     * @class Circle
     * @extends Body
     * @constructor
     */
```