fix typo

CUSTOMIZE.md

@@ -6,7 +6,7 @@ This file will give a brief introduction for:
 
 ## Prepare your own dataset
 
-We would fist give some notations.
+We would first give some notations.
 
 - Prefix: a string that is used to identify your dataset. Suppose it is `vid_custom` here.
 - Split: We need to prepare a `train` and `val` split for training and evaluating our method. When combined with the prefix, they become `vid_custom_train` and `vid_custom_val`.
@@ -108,4 +108,4 @@ Create your model under directory `mega_core/modeling/detector` and register it
 - The field `MODEL.META_ARCHITECTURE` should be your model name.
 - If you feel confused, take a look at other configs.
 
-If you still feel confused with some steps above or some instructions are wrong, please contact me to fix it or make it more clear.
+If you still feel confused with some steps above or some instructions are wrong, please contact me to fix it or make it more clear.