This article is somewhat misleading. Although the image caption says Nominal GDP, the bottom of the image says the GDP is in PPP terms.

AFAIK the most optimistic estimates make India the second largest economy in nominal terms measured by exchange rate no earlier than 2050.

India is expected to be growing pretty fast. But nowhere close to the rate which would be needed for this to be true in currency exchange rate terms. PPP terms really doesn't mean much.