Fix some typos in comments (#4448)

- s/instantion/instantiation/
- s/uninitalized/uninitialized/
- s/costexpr/constexpr/

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

include/fmt/ostream.h

@@ -33,8 +33,8 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-// Generate a unique explicit instantion in every translation unit using a tag
-// type in an anonymous namespace.
+// Generate a unique explicit instantiation in every translation unit using a
+// tag type in an anonymous namespace.
 namespace {
 struct file_access_tag {};
 }  // namespace