Properly generate links when specified source path doesn't end in '/'

Currently, if you specify the source folder on the command line without a trailing slash (e.g. `documenter --source /Users/nj/github/brackets/src ...`, the module links in the generated index file are incorrect (they have an extra /). This pull request strips the beginning slash off the document path stored for each module, which seems to be the assumption other parts of the code are making.