Add resource to tester mindset (#5682)

* Update 101-tester-mindset.md

Adding a reference from ISTQB to the QA Roadmap (Tester Mindset) Section

* Update src/data/roadmaps/qa/content/100-qa-basics/101-tester-mindset.md

Co-authored-by: dsh <daniel.s.holdsworth@gmail.com>

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Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>
Co-authored-by: dsh <daniel.s.holdsworth@gmail.com>

src/data/roadmaps/qa/content/100-qa-basics/101-tester-mindset.md

@@ -14,3 +14,4 @@ Visit the following resources to learn more:
 
 - [The Software Tester’s Mindset](https://softwaretester.careers/the-software-testers-mindset/)
 - [How to Think Like a Tester](https://medium.com/@blakenorrish/how-to-think-like-a-tester-7a174ff6aeaf)
+- [ISTQB® Foundation Level Syllabus 2018](https://www.turkishtestingboard.org/files/FL-Syllabus-2018-GA.pdf)