Update overview_graph.md to use :center symbol

The colon was missing from this symbol, which meant the snippet didn't actually work because `center` is an undefined function.

Author: nschulzke

guides/overview_graph.md

@@ -11,7 +11,7 @@ one thing. When multiple primitives are put together, almost any standard UI can
 For example, the graph below shows the words "Hello World" around them.
 
     @graph Graph.build(font: :roboto, font_size: 24)
-      |> text("Hello World", text_align: center, translate: {300, 300})
+      |> text("Hello World", text_align: :center, translate: {300, 300})
       |> circle(100, stroke: {2, :green}, translate: {300, 300})
 
 In the example above, the first line creates a new graph and assigns two font styles to its root. The next two lines form a pipeline that adds primitives to the root