GDExtension: Fix typo in "GDExtension documentation system" (#11310)

* GDExtension: Fix typo in "GDExtension documentation system"

* Apply suggestions from code review

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Co-authored-by: Hugo Locurcio <[email protected]>

tutorials/scripting/cpp/gdextension_docs_system.rst

@@ -85,7 +85,7 @@ You can set text as bold, italic, underlined, colored, codeblocks etc. by embedd
 
     [b]this text will be shown as bold[/b]
 
-Currently they supported tags for the GDExtension documentation system are:
+Currently, the supported tags for the GDExtension documentation system are:
 
 .. list-table::
   :class: wrap-normal