Update os.h

Author: gabime

include/spdlog/details/os.h

@@ -109,9 +109,9 @@ SPDLOG_API bool create_dir(const filename_t &path);
 // return empty string if field not found
 SPDLOG_API std::string getenv(const char *field);
 
-// Do fsync by FILE object
-// Return true on success
-SPDLOG_API bool fsync(FILE * f);
+// Do fsync by FILE objectpointer.
+// Return true on success.
+SPDLOG_API bool fsync(FILE * fp);
 
 } // namespace os
 } // namespace details