gens(G,i) -> gen(G,i)

[fingolfin]

This matches the conventions in other parts of OSCAR.

Extracted from PR #173 (but also added fix for docs/src/Groups/groups.md and fixed some doc strings)

[codecov]

Codecov Report

Merging #207 (16059aa) into master (27a505a) will decrease coverage by 4.95%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   49.38%   44.43%   -4.96%     
==========================================
  Files          28       28              
  Lines        3922     3601     -321     
==========================================
- Hits         1937     1600     -337     
- Misses       1985     2001      +16     

Impacted Files	Coverage Δ
src/Groups/types.jl	90.32% <ø> (-7.05%)	⬇️
src/Groups/GAPGroups.jl	83.68% <100.00%> (-3.93%)	⬇️
src/Rings/mpoly.jl	40.25% <0.00%> (-6.08%)	⬇️
src/Groups/sub.jl	74.48% <0.00%> (-5.35%)	⬇️
src/Rings/mpoly-local.jl	43.82% <0.00%> (-4.89%)	⬇️
src/Groups/homomorphisms.jl	82.53% <0.00%> (-3.89%)	⬇️
src/Groups/cosets.jl	73.55% <0.00%> (-3.59%)	⬇️
src/Groups/group_constructors.jl	88.57% <0.00%> (-2.92%)	⬇️
... and 13 more