Fix typo.

(Yes this is a breaking change, i don't care, no one uses this yet, and if they do they should have told me!)

src/main/java/de/bluecolored/bluemap/api/markers/POIMarker.java

@@ -138,7 +138,7 @@ public int hashCode() {
      * Creates a Builder for {@link POIMarker}s.
      * @return a new Builder
      */
-    public static Builder toBuilder() {
+    public static Builder builder() {
         return new Builder();
     }