How to acknowledge a colleague's authorship of text in a syllabus
This sort of practice is quite common, and documents of this type are often treated as the "community property" of a department.  This is a lot like authorship on a community wiki, where there is no expectation that the document is the original work of any author and there is the expectation that further authors may freely modify and contribute more text.  It is therefore not plagiarism to use your colleagues notes as a base, any more than it is plagiarism to contribute to a Wikipedia article.

Often, these documents have no copyright or authorship markings, and are simply handled by custom and tradition.  If you want to make the collective agreement more explicit and remove any doubt for those that come after you, then you should probably select a creative commons license (like CC0 or CC-BY) and put a small marking to that effect in the header or footer of the document.  If you choose to formalize it in this way, however, you should explicitly get your colleague's consent as well, because you are moving the document beyond the informal method of relationship-based exchange in which it was given to you.