fix a typo in ostream.h

s/instantion/instantiation/

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

Author: tchaikov

include/fmt/ostream.h

@@ -33,7 +33,7 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-// Generate a unique explicit instantion in every translation unit using a tag
+// Generate a unique explicit instantiation in every translation unit using a tag
 // type in an anonymous namespace.
 namespace {
 struct file_access_tag {};