Bug 1503372 [wpt PR 13790] - Fix link markup in signed-exchange/README.md, a=testonly

Automatic update from web-platform-testsFix link markup in signed-exchange/README.md (#13790)

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wpt-commits: 088f63b9c303e1e3a5be74058c0b81a5d30502f2
wpt-pr: 13790

testing/web-platform/tests/signed-exchange/README.md

@@ -15,7 +15,7 @@ regenerate these files by running `generate-test-sxgs.sh` in the
 `resource` directory before running the tests.
 
 `generate-test-sxgs.sh` requires command-line tools in the
-[webpackage repository][https://github.com/WICG/webpackage].
+[webpackage repository](https://github.com/WICG/webpackage).
 To install them, run:
 ```
 go get -u github.com/WICG/webpackage/go/signedexchange/cmd/...