Fix typo in signgam test messages.

I noticed a typo in the messages from the signgam tests I recently
added.  This patch fixes it.

Tested for x86_64 and x86.

	* math/test-signgam-finite.c (RUN_TESTS): Correct messages about
	calls with argument -0.5.
	* math/test-signgam-finite-c99.c (RUN_TESTS): Likewise.