Ignoring the technicality that $nt$ may not be an integer, the sum is, for $t>0$ $ \sum_{k=1}^{nt} {\textstyle{1\over n}}\bigl(1-{\textstyle{1\over n}}\bigr)^{k-1}= {1\over n}{1-(1-{1\over n})^{(nt+1)-1}\over 1-(1-{1\over n})} =1-(1-{\textstyle{1\over n}})^{nt}. $
So, $\eqalign{ \lim_{n\rightarrow\infty } \sum_{k=1}^{nt} {\textstyle{1\over n}}\bigl(1-{\textstyle{1\over n}}\bigr)^{k-1}&= \lim_{n\rightarrow\infty }\bigl[\,1-(1-\textstyle{1\over n})^{nt}\,\bigr]\cr &=\lim_{n\rightarrow\infty }\bigl[\,1-(1-\textstyle{t\over t n})^{nt}\,\bigr]\cr &=\lim_{k\rightarrow\infty }\bigl[\,1-(1-\textstyle{t\over k})^{k}\,\bigr]\cr &=1-e^{-t}. } $
We really shouldn't ignore the fact that $nt$ may not be an integer, the sum should actually be expressed by $1-(1-{\textstyle{1\over n}})^{\lbrack nt\rbrack}$, say, where $[nt]$ is the integer part of $nt$.
Using the fact that
$\lim\limits_{k\rightarrow\infty}(1+{\textstyle{t\over k}})^{k+\alpha}= \lim\limits_{k\rightarrow\infty}\bigl[\,(1+{\textstyle{t\over k}})^{k }(1+{\textstyle{t\over k}})^{\alpha }\,\bigr]=e^t, $ and the squeeze theorem, one can show $ \lim_{n\rightarrow\infty }\bigl (1-\textstyle{1\over n}\bigr)^{[nt]} =e^{-t}. $