build(java): readme.sh should be executable

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[codecov]

Codecov Report

Merging #378 into master will decrease coverage by 0.58%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #378      +/-   ##
============================================
- Coverage     79.20%   78.62%   -0.59%     
+ Complexity      320      319       -1     
============================================
  Files            21       21              
  Lines          2905     2905              
  Branches        159      159              
============================================
- Hits           2301     2284      -17     
  Misses          537      537              
- Partials         67       84      +17     

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsub/v1/SubscriptionAdminClient.java	57.65% <0.00%> (-5.41%)	52.00% <0.00%> (ø%)
...a/com/google/cloud/pubsub/v1/TopicAdminClient.java	56.64% <0.00%> (-2.32%)	34.00% <0.00%> (ø%)
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	86.89% <0.00%> (-0.25%)	46.00% <0.00%> (-1.00%)

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