Fix graph.vertex_collections docstring

[aMahanna]

It was previously stated that the graph.vertex_collections() method excluded orphan collections, which is not/no longer the case

not sure when this has changed however

[codecov-commenter]

Codecov Report

Merging #292 (f2f6134) into main (744a6e9) will decrease coverage by 0.50%.
Report is 22 commits behind head on main.
The diff coverage is 92.70%.

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@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
- Coverage   99.01%   98.51%   -0.50%     
==========================================
  Files          26       26              
  Lines        3958     4045      +87     
==========================================
+ Hits         3919     3985      +66     
- Misses         39       60      +21     

Files	Coverage Δ
arango/aql.py	95.04% <100.00%> (ø)
arango/client.py	98.41% <ø> (ø)
arango/cluster.py	100.00% <100.00%> (ø)
arango/exceptions.py	100.00% <100.00%> (ø)
arango/executor.py	99.35% <100.00%> (-0.13%)	⬇️
arango/graph.py	100.00% <ø> (ø)
arango/http.py	100.00% <ø> (ø)
arango/job.py	100.00% <100.00%> (ø)
arango/request.py	94.44% <100.00%> (ø)
arango/typings.py	100.00% <100.00%> (ø)
... and 3 more