Originally Posted by
NonCommAlg applying the mean value theorem to the function $\displaystyle f(x)=\ln x, \ 0 < a < b,$ we'll get: $\displaystyle \ln b - \ln a > \frac{b-a}{b}. \ \ \ \ \ (1)$
now suppose $\displaystyle n \geq 2$ and in $\displaystyle (1)$ put $\displaystyle a=\sqrt{n} - 1$ and $\displaystyle b=\sqrt{n}$ to get $\displaystyle \ln \sqrt{n} - \ln(\sqrt{n} - 1) > \frac{1}{\sqrt{n}}.$ therefore:
$\displaystyle \left(1 - \frac{1}{\sqrt{n}} \right)^n < e^{-\sqrt{n}}$ and hence your series is convergent by the comparison test.
(note that $\displaystyle \lim_{n\to\infty} \frac{e^{-\sqrt{n}}}{\frac{1}{n^2}}=\lim_{x\to\infty} \frac{x^4}{e^x}=0$ and thus the series $\displaystyle \sum e^{-\sqrt{n}}$ is convergent by the limit comparison test.)