VL18: (Typo ?) in Summary and Generalisation

In [lecture.tex-18](https://github.com/knowsys/TheoLog/blob/master/Vorlesungen/lecture-18.tex), namely line 244 (see: [here](https://github.com/knowsys/TheoLog/blob/master/Vorlesungen/lecture-18.tex#L244)) is a minor typo, instead of 'ersetzten' it should be 'ersetzen', as also mentioned in the lecture while explaining the procedure (see: [Substitution und Unifikation - 11:26](https://youtu.be/e95nRq_0_MU?t=686))

Open Vorlesungen/lecture-18.tex at line 244 and compare the wording with the linked lecture explanation. Correct the identified spelling in the German text, then review the diff to confirm that only the intended typo changed.

Written by the indexing model from the issue text.