Remove stray gitignore entry

The Chat gitignore that I copied in b38ce55 had a mistake (fixed in
[1]).

[1] ably/ably-chat-swift#353

Author: lawrence-forooghian

.gitignore

@@ -7,7 +7,6 @@ DerivedData/
 .swiftpm/configuration/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .netrc
-docs-coverage-report
 # End of .gitignore created by Swift Package Manager
 
 /node_modules