go/gcexportdata: fix unnecessary plural type definitions of arg

importPath and srcDir are both of string type.

Change-Id: Ia5230bd19ea83bc210cb0b1a50046e4e0ef2accb
Reviewed-on: https://go-review.googlesource.com/42890
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed