Fix go-githubauth link in README.md

[bored-engineer]

The URL is missing the schema so GitHub treats it as a relative link.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.90%. Comparing base (2b8c7fa) to head (7bbe4f5).
Report is 71 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3191 +/- ##
==========================================
- Coverage 97.72% 92.90% -4.82% 
==========================================
Files 153 171 +18 Lines 13390 11548 -1842 ==========================================
- Hits 13085 10729 -2356 - Misses 215 726 +511 - Partials 90 93 +3 

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[gmlewis]

Thank you, @bored-engineer !
LGTM.
Merging.