fix(pkg): pin @octokit/core peerDependency to v5 (#599)

Pin @octokit/core peerDependency to v5

Author: DanCurryy

package.json

@@ -28,7 +28,7 @@
     "@octokit/types": "^12.6.0"
   },
   "peerDependencies": {
-    "@octokit/core": ">=5"
+    "@octokit/core": "5"
   },
   "devDependencies": {
     "@octokit/core": "^5.1.0",