Update README.md

updated output filename to make the purpose more obvious

README.md

@@ -13,7 +13,7 @@ training data. Size also matters: the bigger the model (eg: 70b vs 13b) and the
 the bigger the source file needs to be to make an impact. Multiple input files can be combined if needed; 
 for example:
 ```
-cat technical.txt multilingual.txt wiki.txt >custom.txt
+cat technical.txt multilingual.txt wiki.txt >custom.matrix
 ```
 
 You will find below descriptions for the various input files provided, to help you choose the correct one.