gerrit: use correct URL for GetProjects API call

API doc: https://gerrit-review.googlesource.com/Documentation/rest-api-projects.html#list-projects

Change-Id: I070350948b11d9aa35050eaf1368bbec8d1cad91
GitHub-Last-Rev: 86abe8c6644c08ed462e0d532a106d1b79051d21
GitHub-Pull-Request: golang/build#19
Reviewed-on: https://go-review.googlesource.com/c/build/+/178059
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/gerrit/gerrit.go b/gerrit/gerrit.go
index f554d7f..91d2cdf 100644
--- a/gerrit/gerrit.go
+++ b/gerrit/gerrit.go
@@ -848,7 +848,7 @@
 // GetProjects returns a map of all projects on the Gerrit server.
 func (c *Client) GetProjects(ctx context.Context, branch string) (map[string]*ProjectInfo, error) {
 	mp := make(map[string]*ProjectInfo)
-	err := c.do(ctx, &mp, "GET", fmt.Sprintf("?b=%s&format=JSON", branch))
+	err := c.do(ctx, &mp, "GET", fmt.Sprintf("/projects/?b=%s&format=JSON", branch))
 	return mp, err
 }