[SPARK-26787] Fix standardizeLabels error message in WeightedLeastSquares

Error message falsely states standardization=True is causing a problem, even when standardization=False. The real issue is standardizeLabels=True, which is set automatically in LinearRegression and not currently available in the Public API.

## What changes were proposed in this pull request?

A simple change to an error message. More details here: https://jira.apache.org/jira/browse/SPARK-26787

## How was this patch tested?

This does not change any functionality.

Closes apache#23705 from bscan/bscan-errormsg-1.

Authored-by: bscan <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: bscan

mllib/src/main/scala/org/apache/spark/ml/optim/WeightedLeastSquares.scala

@@ -133,7 +133,7 @@ private[ml] class WeightedLeastSquares(
         return new WeightedLeastSquaresModel(coefficients, intercept, diagInvAtWA, Array(0D))
       } else {
         require(!(regParam > 0.0 && standardizeLabel), "The standard deviation of the label is " +
-          "zero. Model cannot be regularized with standardization=true")
+          "zero. Model cannot be regularized when labels are standardized.")
         instr.logWarning(s"The standard deviation of the label is zero. Consider setting " +
           s"fitIntercept=true.")
       }