[gtkmm-documentation] Fix 2 typos of slot_callbback => slot_callback

[Daniel Boles <dboles src gnome org>]

commit cfcb0f9cd48b7e289cd990f3c3228d3aeb9e6961
Author: Daniel Boles <dboles src gmail com>
Date:   Sun Jan 14 20:48:08 2018 +0000

    Fix 2 typos of slot_callbback => slot_callback

 docs/tutorial/C/index-in.docbook |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index fc6789c..1230f9d 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -9941,7 +9941,7 @@ _WRAP_METHOD(void set_text(const Glib::ustring&amp; text), gtk_entry_set_text)
             to the C function, if the method has one.  Instead the slot itself
             is passed.  The slot parameter name and the glue callback function
             must have been specified with the <literal>slot_name</literal> and
-            <literal>slot_callbback</literal> options respectively.</para>
+            <literal>slot_callback</literal> options respectively.</para>
         </listitem>
     </varlistentry>
 </variablelist>
@@ -10287,7 +10287,7 @@ _WRAP_VFUNC(SizeRequestMode get_request_mode() const, get_request_mode)
             to the C function, if the method has one.  Instead the slot itself
             is passed.  The slot parameter name and the glue callback function
             must have been specified with the <literal>slot_name</literal> and
-            <literal>slot_callbback</literal> options respectively.</para>
+            <literal>slot_callback</literal> options respectively.</para>
         </listitem>
     </varlistentry>
     <varlistentry>