Name examples section in docstrings consistently

[fingolfin]

The Julia style guide suggest to use '# Examples' uniformly, even if
there is just one example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.36%. Comparing base (13a2cd0) to head (26da930).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4376      +/-   ##
==========================================
+ Coverage   84.34%   84.36%   +0.01%     
==========================================
  Files         651      652       +1     
  Lines       86749    86955     +206     
==========================================
+ Hits        73168    73359     +191     
- Misses      13581    13596      +15     

Files with missing lines	Coverage Δ
...imental/BasisLieHighestWeight/src/UserFunctions.jl	92.30% <ø> (ø)
experimental/LieAlgebras/src/LieAlgebra.jl	88.36% <ø> (ø)
experimental/LieAlgebras/src/LieAlgebraModule.jl	88.71% <ø> (ø)
experimental/LieAlgebras/src/RootSystem.jl	93.18% <ø> (ø)
experimental/MatrixGroups/src/MatrixGroups.jl	52.94% <ø> (ø)
experimental/Schemes/src/Resolution_structure.jl	67.31% <ø> (ø)
experimental/Schemes/src/Resolution_tools.jl	88.77% <ø> (ø)
...CoveredProjectiveScheme/CoveredProjectiveScheme.jl	89.18% <ø> (ø)
.../AlgebraicGeometry/Schemes/Divisors/WeilDivisor.jl	77.15% <ø> (ø)
...ry/Schemes/ProjectiveSchemes/Objects/Attributes.jl	87.30% <ø> (ø)
... and 10 more

... and 4 files with indirect coverage changes