CI: fix GHA docs' job - use branches names not tags

[lukaszstolarczuk]

This change is 

[codecov]

Codecov Report

Merging #921 into stable-1.11 will decrease coverage by 0.24%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           stable-1.11     #921      +/-   ##
===============================================
- Coverage        96.12%   95.87%   -0.25%     
===============================================
  Files               48       48              
  Lines             6087     6087              
===============================================
- Hits              5851     5836      -15     
- Misses             236      251      +15     

Flag	Coverage Δ
#tests_clang_debug_cpp17	95.74% <ø> (-0.21%)	⬇️
#tests_gcc_debug	92.35% <ø> (-0.24%)	⬇️

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Impacted Files	Coverage Δ
include/libpmemobj++/mutex.hpp	79.31% <0.00%> (-6.90%)	⬇️
include/libpmemobj++/condition_variable.hpp	76.19% <0.00%> (-2.39%)	⬇️
...ude/libpmemobj++/container/concurrent_hash_map.hpp	93.46% <0.00%> (-1.11%)	⬇️
...j++/container/detail/concurrent_skip_list_impl.hpp	97.99% <0.00%> (-0.14%)	⬇️

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