fix: add whitespace to generateV4SignedPolicy

[frankyn]

Based on changes requested to -> GoogleCloudPlatform/php-docs-samples#1049

[codecov]

Codecov Report

Merging #1136 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1136      +/-   ##
==========================================
- Coverage   99.23%   99.10%   -0.14%     
==========================================
  Files          12       12              
  Lines       11428    11428              
  Branches      525      581      +56     
==========================================
- Hits        11341    11326      -15     
- Misses         87      102      +15     

Impacted Files	Coverage Δ
src/signer.ts	99.28% <0.00%> (-0.48%)	⬇️
src/bucket.ts	99.38% <0.00%> (-0.20%)	⬇️
src/file.ts	99.78% <0.00%> (-0.17%)	⬇️

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