Fixed typos in the documentation of the SBV parser
Fixed typos in the documentation of the SRT parser
Fixed typos in the documentation of the SSA parser
Fixed typos in the documentation of the TTML parser
Fixed typos in the documentation of the TTXT parser
Fixed typos in the documentation of the VTT parser
Fixed typos in the documentation of the XML parser
Fixed typos in the documentation of the JSON parser
Merge pull request #41 from iturralde/parsers-fix-typos

Fixed some typos in the documentation of the parsers