Fix some typos in several scripts usage

[codecov[bot]]

Codecov Report

Merging #26 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   81.89%   81.89%           
=======================================
  Files          44       44           
  Lines        3524     3524           
  Branches      742      742           
=======================================
  Hits         2886     2886           
  Misses        451      451           
  Partials      187      187

Impacted Files	Coverage Δ
pdbtools/pdb_tofasta.py	74.15% <ø> (ø)	:arrow_up:
pdbtools/pdb_delresname.py	80.76% <ø> (ø)	:arrow_up:
pdbtools/pdb_rplresname.py	84% <ø> (ø)	:arrow_up:
pdbtools/pdb_delinsertion.py	86.66% <ø> (ø)	:arrow_up:
pdbtools/pdb_delelem.py	84.41% <ø> (ø)	:arrow_up:
pdbtools/pdb_validate.py	88.31% <ø> (ø)	:arrow_up:

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[JoaoRodrigues]

Thanks!