Fix documentation formatting problem

Author: cheuberg

src/sage/combinat/k_regular_sequence.py

@@ -683,7 +683,7 @@ def forward_differences(self, **kwds):
     def partial_sums(self, include_n=False):
         r"""
         Return the sequence of partial sums of this
-        `k`-regular sequence. That is, the `n`th entry of the result
+        `k`-regular sequence. That is, the `n`-th entry of the result
         is the sum of the first `n` entries in the original sequence.
 
         INPUT: