Let $$\displaystyle y=\lim_{n\rightarrow \infty}\left(\frac{n}{n+1}\right)^n\;,$$ Now taking $\ln$ on both side, we get
$$\displaystyle \ln(y) = \lim_{n\rightarrow \infty}n\cdot \ln\left(\frac{n}{n+1}\right)$$
Now Put $\displaystyle n=\frac{1}{m}\;,$ So $$\displaystyle \ln(y)=\lim_{m\rightarrow 0}\frac{\ln\left(\frac{1}{m+1}\right)}{m} = -\lim_{m\rightarrow 0}\frac{\ln(m+1)}{m} =-1$$
above we have used the formula $$\displaystyle \bullet \lim_{x\rightarrow 0}\frac{\ln(1+x)}{x} =1$$
So we get $$\displaystyle \ln(y) = -1\Rightarrow y=e^{-1}=\frac{1}{e}$$