docs: fix javadoc for Date type

[olavloite]

Fix wrong Java docs for Date type.

Source: https://cloud.google.com/spanner/docs/data-definition-language#date

[codecov]

Codecov Report

Merging #1102 (d5943fe) into master (3fd3902) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1102      +/-   ##
============================================
+ Coverage     84.91%   84.92%   +0.01%     
  Complexity     2725     2725              
============================================
  Files           156      156              
  Lines         14410    14410              
  Branches       1379     1379              
============================================
+ Hits          12236    12238       +2     
+ Misses         1605     1604       -1     
+ Partials        569      568       -1     

Impacted Files	Coverage Δ	Complexity Δ
...r/src/main/java/com/google/cloud/spanner/Type.java	93.20% <ø> (ø)	59.00 <0.00> (ø)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.21% <0.00%> (+0.19%)	75.00% <0.00%> (+1.00%)

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