Update shim.md

Author: DeveloperTryingToCodeLikeOtherOfThem

docs/shim.md

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 ## Shim
 A **shim** is a parser that accesses **only** C++ or the TypeScript Abstraction Layer (TSAL), **not the sandbox environment**, for functions.
 For instance, these shims are **not** able to access properties or anything else in the runtime, which is how they were built. Additionally,
-**only** functions that are exposed in the pxsim namespace, which are exported in the [pxtsim](https://github.com/microsoft/pxt/tree/master/pxtsim/) folder and any PXT repository that contains the sim folder. For instance, the sim folder is in a specific lib folder.
+**only** functions that are exposed in the pxsim namespace, which are implemented in the [pxtsim](https://github.com/microsoft/pxt/tree/master/pxtsim/) folder and any PXT repository that contains the sim folder. For instance, the sim folder is in a specific libs folder in any PXT repository except the core PXT repository.