chore(docs): fix typo

Author: jeertmans

docs/source/reference/api.md

@@ -2,7 +2,7 @@
 
 Manim Slides' API is very limited: it simply consists of two classes, `Slide` and `ThreeDSlide`, which are subclasses of `Scene` and `ThreeDScene` from Manim.
 
-Thefore, we only document here the methods we think the end-user will ever use, not the methods used internally when rendering.
+Therefore, we only document here the methods we think the end-user will ever use, not the methods used internally when rendering.
 
 ```{eval-rst}
 .. autoclass:: manim_slides.Slide