PULL_REQUEST_TEMPLATE.md: "Zlib" should not be capitalized.

(cherry picked from commit fd28dc9d56f42f92275c5e88e89ee60d34a167ea)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,4 @@
-- [ ] I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").
+- [ ] I confirm that I am the author of this code and release it to the SDL project under the zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").
 
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