Fixed minor grammar phrasing

Author: erichschroeter

src/doc/src/reference/build-scripts.md

@@ -19,7 +19,7 @@ build = "build.rs"
 The Rust file designated by the `build` command (relative to the package root)
 will be compiled and invoked before anything else is compiled in the package,
 allowing your Rust code to depend on the built or generated artifacts.
-By default Cargo looks up for `"build.rs"` file in a package root (even if you
+By default Cargo looks for a `"build.rs"` file in a package root (even if you
 do not specify a value for `build`). Use `build = "custom_build_name.rs"` to specify
 a custom build name or `build = false` to disable automatic detection of the build script.