[OpenACC] Fix '#pragma atomic update' typo in 'g++.dg/goacc/template.C'

[OpenACC] Fix '#pragma atomic update' typo in 'g++.dg/goacc/template.C'

    [...]/g++.dg/goacc/template.C:58: warning: ignoring ‘#pragma atomic update’ [-Wunknown-pragmas]
       58 | #pragma atomic update
          |

Small fix-up for r229832 (commit 7a5e4956cc026cba54159d5c764486ac4151db85)
"[openacc] tile, independent, default, private and firstprivate support in
c/++".

	gcc/testsuite/
	* g++.dg/goacc/template.C: Fix '#pragma atomic update' typo.