Delete tip from reference.md

Delete link to GitHub, as requested by engineering.

Author: Amanda Butler

docs/reference/reference.md

@@ -7,5 +7,3 @@ Your application code is written in C++. Your code uses the application programm
 This is the basic architecture of an Mbed board:
 
 <span class="images">![](https://s3-us-west-2.amazonaws.com/mbed-os-docs-images/mbed_internal.png)<span>A sketch of a typical Mbed board's hardware architecture</span></span>
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-<span class="tips">**Tip:** You can explore the [code on GitHub](https://github.com/ARMmbed/mbed-os/tree/mbed-os-5.7).</span>