README: fixed link to example PNG
README: fixed link to example PNG

Former-commit-id: d7465e59e7b6f52b45e03a6525928f70d8f1dc63
lgbouma/astrobase,lgbouma/astrobase,waqasbhatti/astrobase,waqasbhatti/astrobase,lgbouma/astrobase,waqasbhatti/astrobase