[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.
diff --git a/gcc/testsuite/g++.dg/goacc/template.C b/gcc/testsuite/g++.dg/goacc/template.C
index 8bcd2a1..51a3f54 100644
--- a/gcc/testsuite/g++.dg/goacc/template.C
+++ b/gcc/testsuite/g++.dg/goacc/template.C
@@ -55,7 +55,7 @@
 #pragma acc atomic capture
       c = b++;
 
-#pragma atomic update
+#pragma acc atomic update
       c++;
 
 #pragma acc atomic read