Fix example of tf_idf with minDocFreq

## What changes were proposed in this pull request?

The python example for tf_idf with the parameter "minDocFreq" is not properly set up because the same variable is used to transform the document for both with and without the "minDocFreq" parameter.
The IDF(minDocFreq=2) is stored in the variable "idfIgnore" but then it is the original variable "idf" used to transform the "tf" instead of the "idfIgnore".

## How was this patch tested?

Before the results for "tfidf" and "tfidfIgnore" were the same:
tfidf:
(1048576,[1046921],[3.75828890549])
(1048576,[1046920],[3.75828890549])
(1048576,[1046923],[3.75828890549])
(1048576,[892732],[3.75828890549])
(1048576,[892733],[3.75828890549])
(1048576,[892734],[3.75828890549])
tfidfIgnore:
(1048576,[1046921],[3.75828890549])
(1048576,[1046920],[3.75828890549])
(1048576,[1046923],[3.75828890549])
(1048576,[892732],[3.75828890549])
(1048576,[892733],[3.75828890549])
(1048576,[892734],[3.75828890549])

After the fix those are how they should be:
tfidf:
(1048576,[1046921],[3.75828890549])
(1048576,[1046920],[3.75828890549])
(1048576,[1046923],[3.75828890549])
(1048576,[892732],[3.75828890549])
(1048576,[892733],[3.75828890549])
(1048576,[892734],[3.75828890549])
tfidfIgnore:
(1048576,[1046921],[0.0])
(1048576,[1046920],[0.0])
(1048576,[1046923],[0.0])
(1048576,[892732],[0.0])
(1048576,[892733],[0.0])
(1048576,[892734],[0.0])

Author: Maxime Rihouey <maxime.rihouey@gmail.com>

Closes #15503 from maximerihouey/patch-1.

examples/src/main/python/mllib/tf_idf_example.py

@@ -43,7 +43,7 @@ if __name__ == "__main__":
     # In such cases, the IDF for these terms is set to 0.
     # This feature can be used by passing the minDocFreq value to the IDF constructor.
     idfIgnore = IDF(minDocFreq=2).fit(tf)
-    tfidfIgnore = idf.transform(tf)
+    tfidfIgnore = idfIgnore.transform(tf)
     # $example off$
 
     print("tfidf:")