Fix copy-pasta in source/intro.tex

Co-authored-by: Johel Ernesto Guerrero Peña <[email protected]>

Author: frederick-vs-ja

source/intro.tex

@@ -279,7 +279,7 @@
 
 \begin{example}
 If a pointer\iref{dcl.ptr} \tcode{p} whose type is ``pointer to
-class \tcode{B}'' is pointing to a base class subobject class object
+class \tcode{B}'' is pointing to a base class subobject
 of class \tcode{B}, whose most derived object is of class \tcode{D}, derived
 from \tcode{B}\iref{class.derived}, the dynamic type of the
 expression \tcode{*p} is ``\tcode{D}''. References\iref{dcl.ref} are