fix: correct deprecated header warning in asioexec/completion_token.hpp

[keeAzlynth]

// include/asioexec/completion_token.hpp

 #if STDEXEC_MSVC()
 #  pragma message(                                                                                 \
-    "WARNING: The header <exec/asio/completion_token.hpp> is deprecated. Please include <exec/asio/completion_token.hpp> instead.")
+    "WARNING: The header <asioexec/completion_token.hpp> is deprecated. Please include <exec/asio/completion_token.hpp> instead.")
 #else
 #  warning                                                                                         \
-    "The header <exec/asio/completion_token.hpp> is deprecated. Please include <exec/asio/completion_token.hpp> instead."
+    "The header <asioexec/completion_token.hpp> is deprecated. Please include <exec/asio/completion_token.hpp> instead.")
 #endif

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[keeAzlynth]

Thanks for catching this! My oversight — applied your suggestion.