cmd/yacc: fix path in documentation

Change-Id: I367b5a837844e3bee1576c59497d37f5e67c761d
Reviewed-on: https://go-review.googlesource.com/7674Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/yacc/doc.go

@@ -20,7 +20,7 @@ written in C and documented at
 Adepts of the original yacc will have no trouble adapting to this
 form of the tool.
 
-The directory $GOROOT/cmd/yacc/testdata/expr is a yacc program
+The directory $GOROOT/src/cmd/yacc/testdata/expr is a yacc program
 for a very simple expression parser. See expr.y and main.go in that
 directory for examples of how to write and build yacc programs.