[SPARK-26103][SQL][FOLLOW-UP] Use string-interpolation to show the config key.

## What changes were proposed in this pull request?

This is a follow-up of #23169.
We should've used string-interpolation to show the config key in the warn message.

## How was this patch tested?

Existing tests.

Closes #24217 from ueshin/issues/SPARK-26103/s.

Authored-by: Takuya UESHIN <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: ueshin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/StringUtils.scala

@@ -144,7 +144,7 @@ object StringUtils extends Logging {
         logWarning(
           "Truncated the string representation of a plan since it was too long. The " +
             s"plan had length ${length} and the maximum is ${maxLength}. This behavior " +
-            "can be adjusted by setting '${SQLConf.MAX_PLAN_STRING_LENGTH.key}'.")
+            s"can be adjusted by setting '${SQLConf.MAX_PLAN_STRING_LENGTH.key}'.")
         val truncateMsg = if (maxLength == 0) {
           s"Truncated plan of $length characters"
         } else {