Fix ctags language force for c plus plus

[JohanChane]

Filetype for c++ is 'cpp' in vim, however --language-force for c++ is c++ in ctags.

For example, when i reviewed the STL source code, i set the filetype of string file to cpp in vim. And using :BTags, there is a no tags found error. Then i try the below:

# ctags: Unknown language "cpp" in "language-force" option
ctags --language-force=cpp string
# ok
ctags --language-force=c++ string