docs: fix typos in PartitionEvaluator trait documentation (#19631)

## Which issue does this PR close?

Closes #11700

## Rationale for this change

The documentation for the `PartitionEvaluator` trait contains two typos
that could confuse readers.

## What changes are included in this PR?

1. **Line 178**: Removed extraneous 'x' character from the `evaluate`
method documentation
   - Changed `partition.x` to `partition.`

2. **Line 213**: Corrected the RANK example to show proper SQL RANK
behavior
   - Ties should have the same rank value, not sequential values
   - Changed `D  | 5` to `D  | 4`

## Are these changes tested?

Documentation-only change - no code affected.

## Are there any user-facing changes?

No, documentation only.

Co-authored-by: Solari Systems <[email protected]>

Author: SolariSystems

datafusion/expr/src/partition_evaluator.rs

@@ -175,7 +175,7 @@ pub trait PartitionEvaluator: Debug + Send {
     }
 
     /// Evaluate window function on a range of rows in an input
-    /// partition.x
+    /// partition.
     ///
     /// This is the simplest and most general function to implement
     /// but also the least performant as it creates output one row at
@@ -210,7 +210,7 @@ pub trait PartitionEvaluator: Debug + Send {
     ///  A  | 1
     ///  C  | 3
     ///  D  | 4
-    ///  D  | 5
+    ///  D  | 4
     /// ```
     ///
     /// For this case, `num_rows` would be `5` and the