acme: format test data

A test RSA private key was missing "-" in the END delimiter.
While working fine with encoding/pem, other tools like openssl
seem to have more strict requirements.

See
https://github.com/golang/crypto/commit/1777f3ba8c1fed80fcaec3317e3aaa4f627764d2#commitcomment-17343818
for more details.

Change-Id: I5153bb9cf405755d6621f69c1329da8290695eed
Reviewed-on: https://go-review.googlesource.com/22871
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/internal/acme/jws_test.go

@@ -41,7 +41,7 @@ c4clNj/k0Ff5Nm1izPUno4C+0IOl7br39IPmfpSuR6wH/h6iHQDqIeybjxyKvT1G
 N0rRAoGBAKGD+4ZI/E1MoJ5CXB8cDDMHagbE3cq/DtmYzE2v1DFpQYu5I4PCm5c7
 EQeIP6dZtv8IMgtGIb91QX9pXvP0aznzQKwYIA8nZgoENCPfiMTPiEDT9e/0lObO
 9XWsXpbSTsRPj0sv1rB+UzBJ0PgjK4q2zOF0sNo7b1+6nlM3BWPx
------END RSA PRIVATE KEY----
+-----END RSA PRIVATE KEY-----
 `
 
 // This thumbprint is for the testKey defined above.