Fix typos in docstrings

[pvillacorta]

Fix some typos in docstrings that I forgot to change in #408

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.66%. Comparing base (3128e9f) to head (e88ca02).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
- Coverage   90.45%   88.66%   -1.79%     
==========================================
  Files          51       51              
  Lines        2796     2796              
==========================================
- Hits         2529     2479      -50     
- Misses        267      317      +50     

Flag	Coverage Δ
base	88.02% <ø> (ø)
core	77.72% <ø> (-12.66%)	⬇️
files	93.55% <ø> (ø)
komamri	93.98% <ø> (ø)
plots	89.30% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...se/src/datatypes/phantom/motion/ArbitraryMotion.jl	92.50% <ø> (ø)
...IBase/src/datatypes/phantom/motion/SimpleMotion.jl	75.67% <ø> (ø)

... and 7 files with indirect coverage changes