Removes trailing space after git string

Hi,

I believe this adds flexibility to the way the PS1 string is formatted. For example, previously I could not have the following two prompts - one outside a repo and one inside.

    /some/directory :
    /some/directory/repo [master] :

As a space is always added after the git string, I could use either of the following and get one of the above working but not the other:

    PROMPT_COMMAND='__git_ps1 "\w" ": "'
    PROMPT_COMMAND='__git_ps1 "\w" " : "'

With this change you can use the latter and prompts inside and outside a repo would be consistent.

Regards