[MINOR][DOCS] Fix site.SPARK_VERSION pattern in RDD Programming Guide

### What changes were proposed in this pull request?

Fix site.SPARK_VERSION pattern in RDD Programming Guide. I found this when I was developing #47968

### Why are the changes needed?
doc fix

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

doc build

### Was this patch authored or co-authored using generative AI tooling?

no

Closes #47985 from yaooqinn/version.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

docs/rdd-programming-guide.md

@@ -46,7 +46,7 @@ Spark applications in Python can either be run with the `bin/spark-submit` scrip
 
 {% highlight python %}
     install_requires=[
-        'pyspark=={site.SPARK_VERSION}'
+        'pyspark=={{site.SPARK_VERSION}}'
     ]
 {% endhighlight %}