Remove duplicate sccache flag

[mattmassicotte]

Hello!

I'm trying to build the compiler, 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.

I seems like the intention here was to omit it, and explain how to include it if desired.

[AnthonyLatsis]

Thanks.

[AnthonyLatsis]

Please make the commit message more descriptive. Imagine someone else reading it:

• Where was the change made? Could the change have possibly broken anything? A [docs] or docs: prefix is a good enough indication.
• I think "redundant" is more apposite than "duplicate". The latter is normally understood to imply the existence of an exact copy.
• I would spell out the flag as is for clarity: --sccache.
• Why was the flag redundant? The answer is not obvious even if you read the diff. This is a question for the commit description.

[xedin]

I can fix commit message on squash this time.

[xedin]

@swift-ci please smoke test

[xedin]

@swift-ci please smoke test Linux platform

[xedin]

@mattmassicotte Thank you for the contribution!

[mattmassicotte]

Thank you both for accepting this and fixing it up. I just used the in-browser GitHub edit facility to throw this together. The commit message was terrible and, in hindsight, not a mistake I should have made. Will be more careful in the future.