Fix compile-fail syntax in error file

Author: mcarton

src/test/compile-fail/issue-31109.rs

@@ -11,6 +11,7 @@
 fn main() {
     // FIXME(#31407) this error should go away, but in the meantime we test that it
     // is accompanied by a somewhat useful error message.
-    let _: f64 = 1234567890123456789012345678901234567890e-340; //~ ERROR could not evaluate float
-    //~ ERROR unimplemented constant expression: could not evaluate float literal
+    let _: f64 = 1234567890123456789012345678901234567890e-340;
+    //~^ ERROR constant evaluation error
+    //~| unimplemented constant expression: could not evaluate float literal
 }