[docs] NFC: Remove redundant --sccache flag (#75390)

Following along with the Getting Started guide. I noticed that the guide specifically suggests adding the `--sccache` flag if needed, but it is included in the example invocation:

> If you installed and want to use Sccache, add --sccache to the invocation.

This means that the flag is optional and should be removed from the example invocation leaving at the user discretion to use.

Author: mattmassicotte

docs/HowToGuides/GettingStarted.md

@@ -264,7 +264,7 @@ Build the toolchain with optimizations, debuginfo, and assertions, using Ninja:
   ```sh
   utils/build-script --skip-build-benchmarks \
     --swift-darwin-supported-archs "$(uname -m)" \
-    --sccache --release-debuginfo --swift-disable-dead-stripping \
+    --release-debuginfo --swift-disable-dead-stripping \
     --bootstrapping=hosttools
   ```
 - Linux: