call user_name method instead of git_user_name method

[schadenfred]

If the user has not configured git with a github user, the following command:

$ gem bootstrap 

...yields the following error:

Scaffolding gem getsome
ERROR:  While executing gem ... (NameError)
    undefined local variable or method `git_user_name' for #<Gem::Release::Context::Git:0x000055eabe5b5a00>

Gem author's intent appears to have been to call the user_name method defined in line 17 above instead of git_user_name, so I changed the code to reflect that intent. Did not test.

[coveralls]

Coverage remained the same at 96.413% when pulling dd33d16 on schadenfred:master into f4c8e56 on svenfuchs:master.

[pboling]

Ping @svenfuchs this looks safe, corrects an apparent typo.

[PikachuEXE]

Thanks for submitting the request
I don't have perm to release the gem now though :P

[PikachuEXE]

Fix released as 2.0.2, go get it! 🚢

[PikachuEXE]

Test result: