[MINOR] [DOCS] Fix map -> mapToPair in Streaming Java example

[srowen]

Fix map -> mapToPair in Java example. (And zap some unneeded "throws Exception" while here)

[SparkQA]

Test build #28441 has started for PR 4967 at commit ded2bc0.

• This patch merges cleanly.

[SparkQA]

Test build #28441 has finished for PR 4967 at commit ded2bc0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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Test PASSed.