fix(deps): pin @octokit/core peerDependency to v5 (#371)

Author: DanCurryy

package.json

@@ -19,7 +19,7 @@
   "author": "Gregor Martynus (https://twitter.com/gr2m)",
   "license": "MIT",
   "peerDependencies": {
-    "@octokit/core": ">=5"
+    "@octokit/core": "5"
   },
   "devDependencies": {
     "@octokit/core": "^5.0.0",