fix: Correct body in EnterpriseService.InitialConfig

[alexandear]

This PR fixes the body in the EnterpriseService.InitialConfig endpoint. From the documentation:

Before the PR, the request body is:

{"License":"1234-1234","Password":"password"}

After:

{"license":"1234-1234","password":"password"}

The bug was found with the help of musttag linter:

$ ./script/lint.shlinting .github/enterprise_manage_ghes_config_test.go:607:52: the given struct should be annotated with the `json` tag (musttag) assertNilError(t, json.NewDecoder(r.Body).Decode(v)) ^github/timestamp_test.go:132:28: the given struct should be annotated with the `json` tag (musttag) out, err := json.Marshal(tc.data) ^github/timestamp_test.go:199:30: the given struct should be annotated with the `json` tag (musttag) bytes, err := json.Marshal(tc.data) ^3 issues:* musttag: 3

So, the PR enables it as well.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.34%. Comparing base (68b82b5) to head (84ee81d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #3763 +/- ##
=======================================
Coverage 91.34% 91.34% =======================================
Files 187 187 Lines 16702 16702 =======================================
Hits 15257 15257 Misses 1257 1257 Partials 188 188 

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[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.