Correct case for octoposh.dll for use on case sensitive file systems

[mhenderson-so]

When installing the Octoposh module on a case sensitive file system (such as in a Linux VM or WSL) the following error occurs:

Import-Module : The module to process 'OctoPosh.dll', listed in field 'ModuleToProcess/RootModule' of module manifest '/root/.local/share/powershell/Modules/Octoposh/0.6.11/Octoposh.psd1' was not processed because no valid module was found in any module directory.

This is because the correct filename is Octoposh.dll not OctoPosh.dll.

This PR fixes the RootModule to have the correct casing.