embedding.rst: Fix heading underline. NFC (#1840)

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document/core/appendix/embedding.rst

@@ -165,7 +165,7 @@ Modules
 .. _embed-module-instantiate:
 
 :math:`\F{module\_instantiate}(\store, \module, \externval^\ast) : (\store, \moduleinst ~|~ \exception ~|~ \error)`
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+...................................................................................................................
 
 1. Try :ref:`instantiating <exec-instantiation>` :math:`\module` in :math:`\store` with :ref:`external values <syntax-externval>` :math:`\externval^\ast` as imports: