gh-35962: Fix documentation formatting problem: kRegularSequence.partial_sums
    
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Fix the following formatting problem

![image](https://github.com/sagemath/sage/assets/9355569/14eae9ab-
cd41-465d-8139-2ff67c7e2b7c)


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URL: #35962
Reported by: cheuberg
Reviewer(s): Daniel Krenn

Author: Release Manager

src/sage/combinat/k_regular_sequence.py

@@ -1041,7 +1041,7 @@ 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: