cmd/gendb: fix flag defaults

Looks like the usage specification is wrong.

Before:

```
$ gendb -h
Usage of gendb:
  -out string
         (default "Directory to write JSON database to")
  -reports string
         (default "Directory containing toml reports")
```

After:

```
$ gendb -h
Usage of /var/folders/j7/pvz71jxn637dqd96gm80nhwm0000gn/T/go-build330871962/b001/exe/main:
  -out string
        Directory to write JSON database to (default "out")
  -reports string
        Directory containing toml reports (default "reports")
```

Change-Id: I47fd858bdcb19eb22e5aefc7d891497c58e567ba
GitHub-Last-Rev: 07a42f8
GitHub-Pull-Request: #3
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/314550
Reviewed-by: Roland Shoemaker <roland@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

Author: knqyf263

cmd/gendb/main.go

@@ -41,8 +41,8 @@ func matchesCurrent(path string, new []osv.Entry) bool {
 }
 
 func main() {
-	tomlDir := flag.String("reports", "Directory containing toml reports", "")
-	jsonDir := flag.String("out", "Directory to write JSON database to", "")
+	tomlDir := flag.String("reports", "reports", "Directory containing toml reports")
+	jsonDir := flag.String("out", "out", "Directory to write JSON database to")
 	flag.Parse()
 
 	tomlFiles, err := ioutil.ReadDir(*tomlDir)