[MINOR][DOCS] Fix variable name typo in document

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

Fix variable name typo in document

### Why are the changes needed?

For doc

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

Yes

### How was this patch tested?

No

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

No

Closes apache#50443 from Mrhs121/typo.

Authored-by: ShengHuang <huangshengtx@163.com>
Signed-off-by: Kent Yao <yao@apache.org>
(cherry picked from commit 1dfb046)
Signed-off-by: Kent Yao <yao@apache.org>

Author: Mrhs121

docs/spark-connect-overview.md

@@ -352,7 +352,7 @@ Example:
 import org.apache.spark.sql.connect.client.REPLClassDirMonitor
 // Register a ClassFinder to monitor and upload the classfiles from the build output.
 val classFinder = new REPLClassDirMonitor(<ABSOLUTE_PATH_TO_BUILD_OUTPUT_DIR>)
-spark.registerClassFinder(classfinder)
+spark.registerClassFinder(classFinder)
 
 // Upload JAR dependencies
 spark.addArtifact(<ABSOLUTE_PATH_JAR_DEP>)