Fix typo

introduction.md

@@ -47,7 +47,7 @@ working on the training and on the loss gave us the final increase that you can
 We eventually had to deal with the fact that we do not have the same data that OpenAI had during the training of CLIP.
 Thus, we tried to add as much data as possible while keeping the data-quality as high as possible.
 
-We considered three main sources of data:
+We considered four main sources of data:
 
 + [WIT](https://github.com/google-research-datasets/wit) is an image-caption dataset collected from Wikipedia (see, 
 [Srinivasan et al., 2021](https://arxiv.org/pdf/2103.01913.pdf)). We focused on the *Reference Description* captions described in the paper as they are