[GOBBLIN-1363] Fix minor mistakes in the Getting-Started guide

[sekikn]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-1363

Description

• Here are some details about my PR, including screenshots (if applicable):

Fixed the following mistakes/obsolete descriptions in the Getting Started document.

• JDK version that Gobblin requires
• Gradle task name for Findbugs
• Broken links to other pages
• Wrong download URL for avro-tools

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

No additional test, since it's just a documentation fix. Instead, I ran mkdocs locally and made sure the document was fixed as expected.

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[codecov-io]

Codecov Report

Merging #3204 (11f2192) into master (2fa3553) will decrease coverage by 4.28%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3204      +/-   ##
============================================
- Coverage     46.23%   41.95%   -4.29%     
+ Complexity     9788     8931     -857     
============================================
  Files          2019     2019              
  Lines         77439    77439              
  Branches       8602     8602              
============================================
- Hits          35807    32491    -3316     
- Misses        38300    41886    +3586     
+ Partials       3332     3062     -270     

Impacted Files	Coverage Δ	Complexity Δ
.../org/apache/gobblin/util/filters/HiddenFilter.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-5.00%)
...g/apache/gobblin/cluster/HelixMessageSubTypes.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-1.00%)
...gobblin/runtime/mapreduce/GobblinOutputFormat.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-2.00%)
.../gobblin/compaction/suite/CompactionSuiteBase.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-10.00%)
...obblin/compaction/source/CompactionFailedTask.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-3.00%)
...n/cluster/event/ClusterManagerShutdownRequest.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-1.00%)
...blin/compaction/mapreduce/RecordKeyMapperBase.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-1.00%)
...in/compaction/action/CompactionCompleteAction.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-1.00%)
...n/compaction/mapreduce/orc/OrcKeyDedupReducer.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-10.00%)
...n/compaction/suite/CompactionSuiteBaseFactory.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-2.00%)
... and 151 more

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[aplex]

LGTM