dispersion_analysis.py: fix typo
dispersion_analysis.py: fix typo

lokik/sfepy,vlukes/sfepy,lokik/sfepy,sfepy/sfepy,BubuLK/sfepy,rc/sfepy,sfepy/sfepy,BubuLK/sfepy,rc/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,rc/sfepy,vlukes/sfepy,lokik/sfepy,lokik/sfepy