Update rust.json

The regex for the problem matcher is invalid, since the `-` creates a range. But the range is invalid, since it cannot start with `\s`.

rust.json

@@ -22,7 +22,7 @@
                     "code": 3
                 },
                 {
-                    "regexp": "^([\\s->=]*(.*):(\\d*):(\\d*)|.*)$",
+                    "regexp": "^([\\s\\->=]*(.*):(\\d*):(\\d*)|.*)$",
                     "file": 2,
                     "line": 3,
                     "column": 4