[MINOR][SPARKR] spark.glm weightCol should in the signature.

## What changes were proposed in this pull request?
Fix the issue that ```spark.glm``` ```weightCol``` should in the signature.

## How was this patch tested?
Existing tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #14641 from yanboliang/weightCol.

R/pkg/R/mllib.R

@@ -140,7 +140,7 @@ setMethod("spark.glm", signature(data = "SparkDataFrame", formula = "formula"),
 
             jobj <- callJStatic("org.apache.spark.ml.r.GeneralizedLinearRegressionWrapper",
                                 "fit", formula, data@sdf, family$family, family$link,
-                                tol, as.integer(maxIter), weightCol)
+                                tol, as.integer(maxIter), as.character(weightCol))
             return(new("GeneralizedLinearRegressionModel", jobj = jobj))
           })