Fix grammatical error Update CONTRIBUTING.md

[mdqst]

Description:
In the section describing the standard library guide, there was a grammatical issue where a verb was missing, making the sentence incomplete. The sentence in question:

"Documentation for the standard library in the [Standard library developers Guide][std-dev-guide], commonly known as the [std-dev-guide]"

lacked a verb, which made it sound awkward. The revised sentence is now:

"Documentation for the standard library is located in the [Standard Library Developers Guide][std-dev-guide], commonly known as the [std-dev-guide]."

This fix improves the clarity and readability of the documentation. Correct grammar is crucial for ensuring that new contributors can easily understand the instructions.