fix typos

Author: DavidAyotte

src/sage/rings/function_field/drinfeld_modules/drinfeld_module.py

@@ -1580,7 +1580,7 @@ def jk_invariants(self):
         `1 \leqslant k \leqslant r-1` and the values are the
         `j_k`-invariants
 
-        Recall that the `j_k`-invariants of self are defined by:
+        Recall that the `j_k`-invariant of self is defined by:
 
         .. MATH::