Fix typo Github to GitHub

Signed-off-by: KeisukeYamashita <19yamashita15@gmail.com>
Signed-off-by: Tomás Senart <tsenart@gmail.com>

Closes #558

README.md

@@ -42,7 +42,7 @@ Both the library and the CLI are versioned with [SemVer v2.0.0](https://semver.o
 After [v8.0.0](https://github.com/tsenart/vegeta/tree/v8.0.0), the two components
 are versioned separately to better isolate breaking changes to each.
 
-CLI releases are tagged with `cli/vMAJOR.MINOR.PATCH` and published on the [Github releases page](https://github.com/tsenart/vegeta/releases).
+CLI releases are tagged with `cli/vMAJOR.MINOR.PATCH` and published on the [GitHub releases page](https://github.com/tsenart/vegeta/releases).
 As for the library, new versions are tagged with both `lib/vMAJOR.MINOR.PATCH` and `vMAJOR.MINOR.PATCH`.
 The latter tag is required for compatibility with `go mod`.