Fix broken link in README (mozilla#789)

Author: TennyZhuang

README.md

@@ -23,7 +23,7 @@ This is a project initiated by Mozilla to gather code coverage results on Firefo
   - [Example: How to generate .gcda files for a Rust project](#example-how-to-generate-gcda-files-for-a-rust-project)
   - [Generate a coverage report from coverage artifacts](#generate-a-coverage-report-from-coverage-artifacts)
     - [LCOV output](#lcov-output)
-    - [Coveralls/Codecov output](#coverallscodecov-output)
+    - [Coveralls output](#coveralls-output)
     - [grcov with Travis](#grcov-with-travis)
   - [Alternative reports](#alternative-reports)
   - [Hosting HTML reports and using coverage badges](#hosting-html-reports-and-using-coverage-badges)