Fix URL in jsoncpp dummy DEPS.

Was missing the correct prefix.

Bug: 1017514
Change-Id: I05ed5952664e23b632727f7f6c9366bd5bd494ac
Tbr: dpranke@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881992Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709750}

third_party/jsoncpp/DEPS

@@ -14,5 +14,5 @@ vars = {
 }
 
 deps = {
-  'source': '{chromium_git}/open-source-parsers/jsoncpp.git@{jsoncpp_revision}'
+  'source': '{chromium_git}/external/github.com/open-source-parsers/jsoncpp.git@{jsoncpp_revision}'
 }