Fix bug that would have caused every crate build to fail

docs.rs needs build tests :(

```
2020/09/04 23:06:51 [INFO] rustwide::cmd: [stderr] error: Unrecognized option: 'disable-per-crate-search--extern-html-root-url'
```

Author: jyn514

src/docbuilder/rustwide_builder.rs

@@ -581,7 +581,9 @@ impl RustwideBuilder {
 
         let mut env_vars = metadata.environment_variables();
         let rustdoc_flags = env_vars.entry("RUSTDOCFLAGS").or_default();
-        rustdoc_flags.push_str(" --static-root-path / --cap-lints warn --disable-per-crate-search");
+        // WARNING: this *must* end with a space or it will cause rustdoc to give an error
+        rustdoc_flags
+            .push_str(" --static-root-path / --cap-lints warn --disable-per-crate-search ");
         rustdoc_flags.push_str(&rustdoc_flags_extras.join(" "));
 
         let mut command = build