Fix built-in modules link

Closes #240.

README.md

@@ -538,7 +538,7 @@ The challenges with this proposal are intimately tied up with [fallback support]
 
 ### Built-in module remapping
 
-Currently import maps are only specified to remap URL-like import specifiers that correspond to a [fetch scheme](https://fetch.spec.whatwg.org/#fetch-scheme), since those are the only URLs that could result in a module being fetched. If browsers support built-in modules (for example via [this Web IDL pull request](https://fetch.spec.whatwg.org/#fetch-scheme)), then we could consider also allowing built-in module URLs to be remapped. This combines well with [fallback support](#fallback-support) and scopes; see [this section of a previous revision of the explainer](https://github.com/WICG/import-maps/tree/93f94c6dfb268bde3009644b65580fb2fbb98fcf#for-built-in-modules-in-module-import-map-supporting-browsers) for more examples on fallback, and [this section](https://github.com/WICG/import-maps#virtualization-examples) for more use cases and examples surrounding virtualization of built-in modules.
+Currently import maps are only specified to remap URL-like import specifiers that correspond to a [fetch scheme](https://fetch.spec.whatwg.org/#fetch-scheme), since those are the only URLs that could result in a module being fetched. If browsers support built-in modules (which would likely come with their own scheme, such as `std:`), then we could consider also allowing built-in module URLs to be remapped. This combines well with [fallback support](#fallback-support) and scopes; see [this section of a previous revision of the explainer](https://github.com/WICG/import-maps/tree/93f94c6dfb268bde3009644b65580fb2fbb98fcf#for-built-in-modules-in-module-import-map-supporting-browsers) for more examples on fallback, and [this section](https://github.com/WICG/import-maps#virtualization-examples) for more use cases and examples surrounding virtualization of built-in modules.
 
 The primary challenges here are in achieving agreement about built-in modules on the web in the first place. Built-in modules also make other aspects of import maps more complicated, e.g. they tend to necessitate [fallback support](#fallback-support) to enable polyfilling, or [multiple import map support](#multiple-import-map-support) to enable multi-party virtualization.