Add ./ to run x.py

Author: spastorino

src/queries/profiling.md

@@ -20,7 +20,7 @@ address [issue 42678](https://github.com/rust-lang/rust/issues/42678).
 Compile the compiler, up to at least stage 1:
 
 ```
-x.py build library/std
+./x.py build library/std
 ```
 
 ### 2. Run `rustc`, with flags