blob/s3blob: Stop escaping second / in //; it's no longer necessary

[vangent]

Fixes #3246.

[codecov]

Codecov Report

Merging #3251 (65e9599) into master (aeb26ed) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 65e9599 differs from pull request most recent head f4a2dda. Consider uploading reports for the commit f4a2dda to get more accurate results

@@            Coverage Diff             @@
##           master    #3251      +/-   ##
==========================================
- Coverage   77.36%   77.35%   -0.01%     
==========================================
  Files         103      103              
  Lines       13548    13550       +2     
==========================================
+ Hits        10481    10482       +1     
  Misses       2339     2339              
- Partials      728      729       +1     

Impacted Files	Coverage Δ
blob/s3blob/s3blob.go	89.74% <ø> (+0.02%)	⬆️

... and 2 files with indirect coverage changes