[SPARK-28777][PYTHON][DOCS] Fix format_string doc string with the cor…

…rect parameters

### What changes were proposed in this pull request?
The parameters doc string of the function format_string was changed from _col_, _d_ to _format_, _cols_ which is what the actual function declaration states

### Why are the changes needed?
The parameters stated by the documentation was inaccurate

### Does this PR introduce any user-facing change?
Yes.

**BEFORE**
![before](https://user-images.githubusercontent.com/9700541/63310013-e21a0e80-c2ad-11e9-806b-1d272c5cde12.png)

**AFTER**
![after](https://user-images.githubusercontent.com/9700541/63315812-6b870c00-c2c1-11e9-8165-82782628cd1a.png)

### How was this patch tested?
N/A: documentation only
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Closes apache#25506 from darrentirto/SPARK-28777.

Authored-by: darrentirto <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

python/pyspark/sql/functions.py

@@ -1569,8 +1569,9 @@ def format_string(format, *cols):
     """
     Formats the arguments in printf-style and returns the result as a string column.
 
-    :param col: the column name of the numeric value to be formatted
-    :param d: the N decimal places
+    :param format: string that can contain embedded format tags and used as result column's value
+    :param cols: list of column names (string) or list of :class:`Column` expressions to
+        be used in formatting
 
     >>> df = spark.createDataFrame([(5, "hello")], ['a', 'b'])
     >>> df.select(format_string('%d %s', df.a, df.b).alias('v')).collect()