[Doc] Update message-api.rst (#3844)

fix #3812
Co-authored-by: Quan (Andy) Gan <coin2028@hotmail.com>
Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

docs/source/guide/message-api.rst

@@ -96,7 +96,7 @@ features ``ft``, and finally multiply ``ft`` by 2 to get the result
 ``final_ft``. After the call, DGL will clean the intermediate messages ``m``.
 The math formula for the above function is:
 
-.. math::  {final\_ft}_i = 2 * \sum_{j\in\mathcal{N}(i)} ({ft}_j * a_{ij})
+.. math::  {final\_ft}_i = 2 * \sum_{j\in\mathcal{N}(i)} ({ft}_j * a_{ji})
 
 DGL's built-in functions support floating point data types, i.e. the feature must
 be ``half`` (``float16``) /``float``/``double`` tensors.