Update README.md

Author: tamaskis

README.md

@@ -21,7 +21,7 @@ Creates high-resolution renderings of the Earth and the major celestial bodies i
    - `position` → specifies the position of the center of the celestial body 
    - `reference_plane` → specifies which reference plane the celestial body is drawn with respect to. If specified as `equatorial`, the reference plane is taken to be the equatorial plane of the celestial body. If specified as `ecliptic`, the celestial body will be tilted by the obliquity (i.e. the angle between the ecliptic plane and the equatorial plane).
    - `theta` → specifies the rotation angle about the 3rd axis of the celestial body (in degrees)
-   - `units` → specifies the units the celestial body should be drawn in. Units available are `'AU'`, `'ft'`, `'km'`, `m`, `'mi'`, and `'nmi`.
+   - `units` → specifies the units the celestial body should be drawn in. Units available are `'AU'`, `'ft'`, `'km'`, `'m'`, `'mi'`, and `'nmi'`.
    - `transparency` → specifies how transparent the celestial body is (1 for 100% transparency, 0 for 100% opacity)
 
 **NOTE:** All fields of `opts` do NOT have to be defined; when a field is left undefined, the rest of the plot settings are set to default values. Additionally, `color`, `line_width`, and `line_style` are only relevant when plotting the Earth's coastlines.