Fix indented display for multi-child nodes

[alamb]

Which issue does this PR close?

Closes #345

Rationale for this change

When generating display output, the indent level of nodes with more than one child input was not correct.

Note the code with the bug has not yet been released yet

[codecov-commenter]

Codecov Report

Merging #358 (d9b1b85) into master (aa26112) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   75.84%   75.86%   +0.01%     
==========================================
  Files         143      143              
  Lines       23809    23824      +15     
==========================================
+ Hits        18059    18075      +16     
+ Misses       5750     5749       -1     

Impacted Files	Coverage Δ
datafusion/src/physical_plan/display.rs	86.66% <100.00%> (+2.05%)	⬆️
datafusion/tests/sql.rs	99.89% <100.00%> (+<0.01%)	⬆️
datafusion/src/physical_plan/mod.rs	82.35% <0.00%> (-2.36%)	⬇️
datafusion/src/physical_plan/hash_join.rs	86.05% <0.00%> (+0.47%)	⬆️

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[alamb]

fyi @andygrove

[andygrove]

🎉

[Dandandan]

💯